do_change_type(): refuse to operate on unmounted/not ours mounts

Ensure that propagation settings can only be changed for mounts located
in the caller's mount namespace. This change aligns permission checking
with the rest of mount(2).

Reviewed-by: Christian Brauner <[e-mail]>
Fixes: 07b20889e305 ("beginning of the shared-subtree proper")
Reported-by: "Orlando, Noah" <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/namespace.c

···
 		return -EINVAL;
 
 	namespace_lock();
+	if (!check_mnt(mnt)) {
+		err = -EINVAL;
+		goto out_unlock;
+	}
 	if (type == MS_SHARED) {
 		err = invent_group_ids(mnt, recurse);
 		if (err)