tcp: fix tcp_xmit_retransmit_queue() after rbtree introduction

I tried to hard avoiding a call to rb_first() (via tcp_rtx_queue_head)
in tcp_xmit_retransmit_queue(). But this was probably too bold.

Quoting Yuchung :

We might miss re-arming the RTO if tp->retransmit_skb_hint is not NULL.
This can happen when RACK marks the first packet lost again and resets
tp->retransmit_skb_hint for example (tcp_rack_mark_skb_lost())

Fixes: 75c119a ("tcp: implement rb-tree based retransmit queue")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -2921,19 +2921,16 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 void tcp_xmit_retransmit_queue(struct sock *sk)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);
-	struct sk_buff *skb, *rtx_head = NULL, *hole = NULL;
+	struct sk_buff *skb, *rtx_head, *hole = NULL;
 	struct tcp_sock *tp = tcp_sk(sk);
 	u32 max_segs;
 	int mib_idx;
 
 	if (!tp->packets_out)
 		return;
 
-	skb = tp->retransmit_skb_hint;
-	if (!skb) {
-		rtx_head = tcp_rtx_queue_head(sk);
-		skb = rtx_head;
-	}
+	rtx_head = tcp_rtx_queue_head(sk);
+	skb = tp->retransmit_skb_hint ?: rtx_head;
 	max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
 	skb_rbtree_walk_from(skb) {
 		__u8 sacked;