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.

MFC after: 2 weeks
Reviewed by: pfg
Differential Revision: https://reviews.freebsd.org/D34753