tcp: exit if nothing to retransmit on RTO timeout

jira LE-3201
Rebuild_History Non-Buildable kernel-rt-4.18.0-553.22.1.rt7.363.el8_10
commit-author Yuchung Cheng <ycheng@google.com>
commit 88f8598

Previously TCP only warns if its RTO timer fires and the
retransmission queue is empty, but it'll cause null pointer
reference later on. It's better to avoid such catastrophic failure
and simply exit with a warning.

	Signed-off-by: Yuchung Cheng <ycheng@google.com>
	Signed-off-by: Eric Dumazet <edumazet@google.com>
	Reviewed-by: Neal Cardwell <ncardwell@google.com>
	Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
	Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 88f8598)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

net/ipv4/tcp_timer.c

@@ -449,10 +449,8 @@ void tcp_retransmit_timer(struct sock *sk)
 		 */
 		return;
 	}
-	if (!tp->packets_out)
-		goto out;
-
-	WARN_ON(tcp_rtx_queue_empty(sk));
+	if (!tp->packets_out || WARN_ON_ONCE(tcp_rtx_queue_empty(sk)))
+		return;
 
 	tp->tlp_high_seq = 0;