mounts: initialize root before dumping mount namespace

Currently it's initialized for the root mount namespace, but we are
going to dump nested mount namespaces.

It's used in open_mountpoint(), which is used in dump_tmpfs() and in
other callbacks.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -821,6 +821,9 @@ int dump_mnt_ns(int ns_pid, int ns_id)
 	if (img_fd < 0)
 		return -1;
 
+	if (mntns_collect_root(ns_pid))
+		goto err;
+
 	pm = parse_mountinfo(ns_pid);
 	if (!pm) {
 		pr_err("Can't parse %d's mountinfo\n", ns_pid);