don't bother with propagate_mnt() unless the target is shared

If the dest_mnt is not shared, propagate_mnt() does nothing -
there's no mounts to propagate to and thus no copies to create.
Might as well don't bother calling it in that case.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -1653,16 +1653,14 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 		err = invent_group_ids(source_mnt, true);
 		if (err)
 			goto out;
-	}
 		err = propagate_mnt(dest_mnt, dest_mp, source_mnt, &tree_list);
 		if (err)
 			goto out_cleanup_ids;
-
 		lock_mount_hash();
-
-	if (IS_MNT_SHARED(dest_mnt)) {
 		for (p = source_mnt; p; p = next_mnt(p, source_mnt))
 			set_mnt_shared(p);
+	} else {
+		lock_mount_hash();
 	}
 	if (parent_path) {
 		detach_mnt(source_mnt, parent_path);
@@ -1685,7 +1683,6 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 	return 0;
 
  out_cleanup_ids:
-	if (IS_MNT_SHARED(dest_mnt))
 	cleanup_group_ids(source_mnt, NULL);
  out:
 	return err;