mount: Prohibit second binfmt_misc superblock

This is need to catch the moment when binfmt_misc in mainline kernel is virtualized
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/mount.c

@@ -1461,6 +1461,7 @@ err:
 
 static int binfmt_misc_dump(struct mount_info *pm)
 {
+	static bool dumped = false;
 	struct cr_img *img = NULL;
 	struct dirent *de;
 	DIR *fdir = NULL;
@@ -1470,6 +1471,12 @@ static int binfmt_misc_dump(struct mount_info *pm)
 	if (ret <= 0)
 		return ret;
 
+	if (dumped) {
+		pr_err("Second binfmt_misc superblock\n");
+		return -1;
+	}
+	dumped = true;
+
 	fd = open_mountpoint(pm);
 	if (fd < 0)
 		return fd;