tcp: refactor tcp_retransmit_timer()

jira LE-3201
Rebuild_History Non-Buildable kernel-rt-4.18.0-553.22.1.rt7.363.el8_10
commit-author Eric Dumazet <edumazet@google.com>
commit 0d580fb

It appears linux-4.14 stable needs a backport of commit
88f8598 ("tcp: exit if nothing to retransmit on RTO timeout")

Since tcp_rtx_queue_empty() is not in pre 4.15 kernels,
let's refactor tcp_retransmit_timer() to only use tcp_rtx_queue_head()

I will provide to stable teams the squashed patches.

	Signed-off-by: Eric Dumazet <edumazet@google.com>
	Cc: Willem de Bruijn <willemb@google.com>
	Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
	Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
	Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 0d580fb)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

net/ipv4/tcp_timer.c

@@ -437,6 +437,7 @@ void tcp_retransmit_timer(struct sock *sk)
 	struct net *net = sock_net(sk);
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct request_sock *req;
+	struct sk_buff *skb;
 
 	req = rcu_dereference_protected(tp->fastopen_rsk,
 					lockdep_sock_is_held(sk));
@@ -449,7 +450,12 @@ void tcp_retransmit_timer(struct sock *sk)
 		 */
 		return;
 	}
-	if (!tp->packets_out || WARN_ON_ONCE(tcp_rtx_queue_empty(sk)))
+
+	if (!tp->packets_out)
+		return;
+
+	skb = tcp_rtx_queue_head(sk);
+	if (WARN_ON_ONCE(!skb))
 		return;
 
 	tp->tlp_high_seq = 0;
@@ -483,7 +489,7 @@ void tcp_retransmit_timer(struct sock *sk)
 			goto out;
 		}
 		tcp_enter_loss(sk);
-		tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1);
+		tcp_retransmit_skb(sk, skb, 1);
 		__sk_dst_reset(sk);
 		goto out_reset_timer;
 	}