Add nodefs.Mount - utility to mount root over mountpoint with given
options. We already had nodefs.MountRoot, but that was taking only
nodefs.Options and there was no way to pass fuse.MountOptions in. The
new utility accepts both fuse.MountOptions and nodefs.Options, each
covering their level.

This should be generally useful(*), as well as it will be used in a next
patch in TestFopenKeepCache where fuse.MountOptions.PreciseDataCacheControl
will need to be used.

(*) see e.g. https://lab.nexedi.com/kirr/wendelin.core/blob/8f497094/wcfs/misc.go#L258
as example that users unroll their Mount versions to be able to pass in
fuse.MountOptions.

CAP_AUTO_INVAL_DATA is capability of kernel, but from the point of view
of a filesystem it is not a capability, but a behaviour request: if set,
it requests to kernel - filesystem client - to perform data cache
invalidations based on heuristics. Current heuristic is to drop data
cache for a file if kernel sees file's mtime being changed:

	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/fuse/inode.c?id=v5.0-0-g1c163f4c7b3f#n238
	https://git.kernel.org/linus/eed2179efe

However this could be unwanted behaviour if filesystem is careful to
explicitly invalidate local file cache: despite filesystem attempts to
preciously keep data cache, the whole cache of the file is unnecessarily
dropped.

-> To fix add a new mount options for filesystems to indicate they are
careful with respect to data cache invalidations and thus should be
fully responsible for invalidating data cache. Teach go-fuse to not send
CAP_AUTO_INVAL_DATA in this mode to kernel on FUSE handshake.

Note, as of upcoming Linux 5.1 (estimated to be released mid 2019), FUSE
client in kernel still automatically and unconditionally drops whole
data cache of a file if its sees size change. Kernel and go-fuse patches
to fix that are here:

	https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git/commit/?id=ad2ba64dd489
	https://github.com/hanwen/go-fuse/pull/273

The kernel patch is likely to enter mainline kernel when 5.2 merge
window opens. We'll remove XXX in a follow-up go-fuse patch that adds
support for CAP_EXPLICIT_INVAL_DATA.

FOPEN_STREAM, together with FOPEN_NONSEEKABLE must be used on
stream-like file handles that provide both read and write to avoid
hitting deadlock in the kernel.

Please see the following kernel patch for details on how the deadlock
can happen:

	git.kernel.org/linus/10dce8af3422

Adding FOPEN_STREAM to kernel FUSE is in fuse.git#for-next now and is likely
to enter mainline kernel when 5.2 merge window opens way or another:

	https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git/commit/?id=bbd84f33652
	https://lore.kernel.org/linux-fsdevel/CAHk-=whQQdoQsgEx1vO7OkfPDcV5hurPnMRLgzfXAPN63n5Sbg@mail.gmail.com/
	https://lore.kernel.org/linux-fsdevel/CAHk-=wjEOyba5As1PEMk6RitNVOJH9oJ_Jbg4y=5B0fcX1iKGw@mail.gmail.com/
	https://lore.kernel.org/linux-fsdevel/CAHk-=wgh234SyBG810=vB360PCzVkAhQRqGg8aFdATZd+daCFw@mail.gmail.com/
	https://lore.kernel.org/linux-fsdevel/20190424183012.GB3798@deco.navytux.spb.ru/

Here is example for FOPEN_STREAM usage:

	https://lab.nexedi.com/kirr/wendelin.core/blob/7783ecf4/wcfs/misc.go#L327-335
	https://lab.nexedi.com/kirr/wendelin.core/blob/7783ecf4/wcfs/misc.go#L276-428

When kernel sends WRITE request it sends with it file handle, offset,
size etc. We were not printing all this. Compare e.g. debug output
sample for TestUtimesNano before and after the patch.

before:

	rx 8: CREATE i1 {0100600 [CREAT,TRUNC,WRONLY,0x8000] (00)} ["hello.txt"] 10b
	tx 8:     OK, {i3 g2 {M0100600 SZ=0 L=1 1000:1000 B0*4096 i0:733462 A 1556038355.426630 M 1556038355.426630 C 1556038355.426630} &{2 0 0}}
	rx 9: GETXATTR i3 {sz 0} ["security.capability"] 20b
	tx 9:     61=no data available
	rx 10: WRITE i3  3b							<-- NOTE
	tx 10:     OK

after:

	rx 8: CREATE i1 {0100600 [WRONLY,CREAT,TRUNC,0x8000] (00)} ["hello.txt"] 10b
	tx 8:     OK, {i3 g2 {M0100600 SZ=0 L=1 1000:1000 B0*4096 i0:736300 A 1556038379.359197 M 1556038379.359197 C 1556038379.359197} &{2 0 0}}
	rx 9: GETXATTR i3 {sz 0} ["security.capability"] 20b
	tx 9:     61=no data available
	rx 10: WRITE i3 {Fh 2 [0 +3)  L 0 WRONLY,NONBLOCK,0x8000}  3b		<-- NOTE
	tx 10:     OK

The new naming makes clear that this is the One True API for Go-FUSE

There is a hang that appears when enabling CAP_PARALLEL_DIROPS on Linux
4.15.0: https://github.com/hanwen/go-fuse/issues/281

The hang was originally triggered by gvfs-udisks2-volume-monitor. This
test emulates what gvfs-udisks2-volume-monitor does.

On 4.15.0 kernels, the test will get stuck, and after 120 seconds you
get a kernel backtrace like this:

[ 1813.463679] INFO: task nodefs.test:2357 blocked for more than 120 seconds.
[ 1813.463685]       Not tainted 4.15.0-45-generic #48~16.04.1-Ubuntu
[ 1813.463687] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 1813.463689] nodefs.test     D    0  2357   2311 0x00000004
[ 1813.463691] Call Trace:
[ 1813.463709]  __schedule+0x3d6/0x8b0
[ 1813.463712]  schedule+0x36/0x80
[ 1813.463714]  schedule_preempt_disabled+0xe/0x10
[ 1813.463716]  __mutex_lock.isra.2+0x2ae/0x4e0
[ 1813.463720]  ? ___slab_alloc+0x223/0x4e0
[ 1813.463722]  ? _cond_resched+0x1a/0x50
[ 1813.463724]  __mutex_lock_slowpath+0x13/0x20
[ 1813.463725]  ? __mutex_lock_slowpath+0x13/0x20
[ 1813.463727]  mutex_lock+0x2f/0x40
[ 1813.463729]  fuse_lock_inode+0x2a/0x30
[ 1813.463732]  fuse_lookup+0x31/0x140
[ 1813.463735]  ? d_alloc_parallel+0xc1/0x4c0
[ 1813.463738]  fuse_atomic_open+0x6d/0xf0
[ 1813.463740]  path_openat+0xc5d/0x13f0
[ 1813.463744]  do_filp_open+0x99/0x110
[ 1813.463747]  ? __check_object_size+0xfc/0x1a0
[ 1813.463749]  ? __alloc_fd+0x46/0x170
[ 1813.463752]  do_sys_open+0x12d/0x290
[ 1813.463754]  ? do_sys_open+0x12d/0x290
[ 1813.463756]  SyS_openat+0x14/0x20
[ 1813.463759]  do_syscall_64+0x73/0x130
[ 1813.463762]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2

The old code searched for the first non-null byte from the end of
the slice. This assumes that all bytes after the name are initialized
to zero, which does not hold true on Linux 4.15.

Instead, search for the first null byte from the start of the slice,
which is guaranteed by man(3) readdir.

Fixes https://github.com/hanwen/go-fuse/issues/287

This reduces the debug output from 25k lines to ~1k

Appeases the race detector.

This currently fails on Linux 5.0 and may be related to
https://github.com/hanwen/go-fuse/issues/287 .

1 jakob@brikett:~/go/src/github.com/hanwen/go-fuse/nodefs$ go test
21:42:47.356529 writer: Write/Writev failed, err: 2=no such file or directory. opcode: RELEASE
21:42:47.598309 writer: Write/Writev failed, err: 22=invalid argument. opcode: READDIRPLUS
21:42:47.604424 writer: Write/Writev failed, err: 22=invalid argument. opcode: READDIRPLUS
21:42:47.606073 writer: Write/Writev failed, err: 22=invalid argument. opcode: READDIRPLUS
--- FAIL: TestReadDirStress (0.36s)
    simple_test.go:270: goroutine 2 iteration 5: readdirent: input/output error
    simple_test.go:270: goroutine 1 iteration 9: readdirent: input/output error
    simple_test.go:270: goroutine 3 iteration 10: readdirent: input/output error
    simple_test.go:43: /usr/bin/fusermount: entry for /tmp/TestReadDirStress639795934/mnt not found in /etc/mtab
         (code exit status 1)

FAIL
exit status 1
FAIL	github.com/hanwen/go-fuse/nodefs	0.994s

Access file descriptors under lock, and set to -1 on close. This
avoids confusing errors if Close() is doubly called

This is based on the new nodefs API, and tests the new API for writing
a full-fledged r/w filesystem.