mount: don't apply superblock flags for external mounts

Part-of: Fix a few issues to dump/restore Docker containers with userns
travis-ci: success for Fix a few issues to dump/restore Docker containers with userns
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/mount.c

@@ -1725,7 +1725,7 @@ static int do_new_mount(struct mount_info *mi)
 		goto out;
 	}
 
-	if (remount_ro && mount(NULL, mi->mountpoint, tp->name,
+	if (!mi->is_ns_root && !mi->external && remount_ro) {
 				     MS_REMOUNT | MS_RDONLY, NULL)) {
 		pr_perror("Unable to apply mount options");
 		return -1;