fuse: check s_root when destroying sb

Checking "fm" works because currently sb->s_fs_info is cleared on error
paths; however, sb->s_root is what generic_shutdown_super() checks to
determine whether the sb was fully initialized or not.

This change will allow cleanup of sb setup error paths.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/inode.c

@@ -1747,7 +1747,7 @@ static void fuse_sb_destroy(struct super_block *sb)
 	struct fuse_mount *fm = get_fuse_mount_super(sb);
 	bool last;
 
-	if (fm) {
+	if (sb->s_root) {
 		last = fuse_mount_remove(fm);
 		if (last)
 			fuse_conn_destroy(fm);

fs/fuse/virtio_fs.c

@@ -1394,7 +1394,7 @@ static void virtio_kill_sb(struct super_block *sb)
 	bool last;
 
 	/* If mount failed, we can still be called without any fc */
-	if (fm) {
+	if (sb->s_root) {
 		last = fuse_mount_remove(fm);
 		if (last)
 			virtio_fs_conn_destroy(fm);