fuse: verify nlink

commit c634da71 upstream.

When adding a new hard link, make sure that i_nlink doesn't overflow.

Fixes: ac45d613 ("fuse: fix nlink after unlink")
Cc: <stable@vger.kernel.org> # v3.4
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>