As seen in debug logs like shown below, concurrent
FORGETs and inode number reuse could make use give
an Inode to the kernel that has already been deleted
from the nodes[] map.

With this change, I can no langer trigger a panic
in xfstests generic/007.

Fixes https://github.com/hanwen/go-fuse/issues/324

Nodes that appear to change type are not handled yet
(actually caused by inode number reuse), but this is
a separate problem.

Debug log:

  17:09:07.651735 rx 84444: CREATE i13632136 {0100644 [WRONLY,TRUNC,CREAT,0x8000] (022)} ["XXXXXXXXXXXX.38"] 16b
  17:09:07.651790 tx 84444:     OK, {i13632483 g1 {M0100644 SZ=0 L=1 0:0 B0*4096 i0:13632483 A 1572192547.650470 M 1572192547.650470 C 1572192547.650470} &{7 0 0}}
  [...]
  17:09:07.684439 rx 87988: CREATE i13632119 {0100644 [CREAT,TRUNC,WRONLY,0x8000] (022)} ["XXXXXXXXXXXX.37"] 16b
  [...]
  17:09:07.684472 rx 87992: FORGET i13632483 {Nlookup=1}
  17:09:07.684483 tx 87988:     OK, {i13632483 g1 {M0100644 SZ=0 L=1 0:0 B0*4096 i0:13632483 A 1572192547.683470 M 1572192547.683470 C 1572192547.683470} &{8 0 0}}
  [...]
  17:09:07.684557 rx 88006: FLUSH i13632483 {Fh 8}
  17:09:07.684560 unknown node 13632483
  17:09:07.684564 tx 88002:     OK,  "XXXXXXXXXXXX.830"
  panic: unknown node 13632483

Change-Id: Ibb1a7a6a459d2e4133a4e512e0816e8ed950a8d7