If kernel internally serializes lookup requests for a directory, the
test will deadlock, becaue it tries to issue two such request and pause
lookup handlers, untill both requests come in.

For example here is how this test hangs on Ubuntu Xenial 16.04:

kirr@kubu:~/t/go-fuse$ go test -count=1 -v -run TestNodeParallelLookup ./fuse/test/
=== RUN   TestNodeParallelLookup
18:24:38.494651 rx 1: INIT i0 {7.23 Ra 0x20000 POSIX_LOCKS,ATOMIC_O_TRUNC,EXPORT_SUPPORT,DONT_MASK,FLOCK_LOCKS,READDIRPLUS_AUTO,ASYNC_DIO,WRITEBACK_CACHE,SPLICE_WRITE,SPLICE_READ,IOCTL_DIR,AUTO_INVAL_DATA,READDIRPLUS,NO_OPEN_SUPPORT,ASYNC_READ,BIG_WRITES,SPLICE_MOVE}
18:24:38.494713 tx 1:     OK, {7.23 Ra 0x20000 ASYNC_READ,BIG_WRITES,AUTO_INVAL_DATA,READDIRPLUS,NO_OPEN_SUPPORT 0/0 Wr 0x10000 Tg 0x0}
18:24:38.495351 rx 2: LOOKUP i1 [".go-fuse-epoll-hack"] 20b
18:24:38.495467 tx 2:     OK, {i18446744073709551615 g0 tE=0s tA=0s {M0100644 SZ=0 L=1 0:0 B0*0 i0:18446744073709551615 A 0.000000 M 0.000000 C 0.000000}}
18:24:38.495530 rx 3: OPEN i18446744073709551615 {O_RDONLY,0x8000}
18:24:38.495575 tx 3:     OK, {Fh 18446744073709551615 }
18:24:38.495624 rx 4: POLL i18446744073709551615
18:24:38.495659 tx 4:     38=function not implemented
18:24:38.495706 rx 5: FLUSH i18446744073709551615 {Fh 18446744073709551615}
18:24:38.495741 tx 5:     34=numerical result out of range
18:24:38.495812 rx 6: RELEASE i18446744073709551615 {Fh 18446744073709551615 0x8000  L0}
18:24:38.495849 tx 6:     34=numerical result out of range
18:24:38.495904 rx 7: LOOKUP i1 ["world"] 6b
18:24:38.495957 I: <- lookup "world"
< hang >

Amends: 17c0c400 (nodefs: Allow for several Lookup requests to be served simultaneously)
Updates: https://github.com/hanwen/go-fuse/issues/261

Change-Id: I5d0a76d1190af805351326c378f69cf8ff781a37