tty: Prepare for no parasite_ctl

SCM-dumping code won't provide the ctl for TTY dump. To
make this work we'll have to peek required tty info from
current (see the SCM patch), but I doubt that anyone
sends TTYs via sockets ... so keep this in TODO list.
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/tty.c

@@ -1760,6 +1760,11 @@ static int dump_tty_info(int lfd, u32 id, const struct fd_parms *p, struct tty_d
 
 	int ret = -1;
 
+	if (!p->fd_ctl) {
+		pr_err("No CTL for TTY dump, likely SCM case\n");
+		return -1;
+	}
+
 	/*
 	 * Make sure the structures the system provides us
 	 * correlates well with protobuf templates.