mount: apply superblock flags to nested ns roots

Before this change we didn't apply sb-flags if we mount the root mount of
non-root mntns. There is no point in it, if we got to do_new_mount this root
mount is not external bind, so we won't change sb-flags on host if we change it
for this mount. So we just loose sb-flags on some regular container mount for
no reason. Fix it.

Cherry-picked from Virtuozzo criu:
https://src.openvz.org/projects/OVZ/repos/criu/commits/e7ffe4c60

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

criu/mount.c

@@ -2213,7 +2213,7 @@ static int do_new_mount(struct mount_info *mi)
 		goto out;
 	}
 
-	if (!mi->is_ns_root && remount_ro) {
+	if (remount_ro) {
 		int fd;
 
 		fd = open(mi->mountpoint, O_PATH);