fuse: don't unhash root

The root inode is assumed to be always hashed.  Do not unhash the root
inode even if it is marked BAD.

Fixes: 5d069dbe ("fuse: fix bad inode")
Cc: <stable@vger.kernel.org> # v5.11
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>