Taking the treeLock after inodeMap.Forget() exposed a
small window where the Inode that is about to be forgotten
could be handed out to the kernel under a new node ID.

Testing with fsstress for 1.000+ iterations showed that the
race can actually be hit, and causes "No such file or directory"
errors.

With the lock taken earlier, loopbackfs passes 10.000+ fsstress
iterations.

Change-Id: Ic02ae757d9a726ece01e8f0b3f0ff877f8e2dbc3