mount: don't close a namespace descriptor two times

CID null (#1 of 1): Double close (USE_AFTER_FREE)
10. double_close: Calling "restore_ns(int, struct ns_desc *)" closes
handle "ns_old" which has already been closed.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -455,8 +455,10 @@ static DIR *open_mountpoint(struct mount_info *pm)
 	if (fd < 0)
 		goto out;
 
-	if (restore_ns(ns_old, &mnt_ns_desc))
+	if (restore_ns(ns_old, &mnt_ns_desc)) {
+		ns_old = -1;
 		goto out;
+	}
 
 	return __open_mountpoint(pm, fd);;
 out: