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.