fusefs: fix two bugs regarding VOP_RECLAIM of the root inode

fusefs: fix two bugs regarding VOP_RECLAIM of the root inode

* We never send FUSE_LOOKUP for the root inode, since its inode number
  is hard-coded to 1.  Therefore, we should not send FUSE_FORGET for it,
  lest the server see its lookup count fall below 0.

* During VOP_RECLAIM, if we are reclaiming the root inode, we must clear
  the file system's vroot pointer.  Otherwise it will be left pointing
  at a reclaimed vnode, which will cause future VOP_LOOKUP operations to
  fail.  Previously we only cleared that pointer during VFS_UMOUNT.  I
  don't know of any real-world way to trigger this bug.

Reviewed by:	pfg
Differential Revision: https://reviews.freebsd.org/D34753

(cherry picked from commit 32273253667b941c376cf08383006b3a0cbc5ca2)