tcp: remove TCP_SKB_CB(skb)->when

After commit 740b0f1841f6 ("tcp: switch rtt estimations to usec resolution"),
we no longer need to maintain timestamps in two different fields.

TCP_SKB_CB(skb)->when can be removed, as same information sits in skb_mstamp.stamp_jiffies

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index df90cd1..a339e7b 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -135,10 +135,9 @@
 	if (!inet_csk(sk)->icsk_retransmits)
 		return false;
 
-	if (unlikely(!tcp_sk(sk)->retrans_stamp))
-		start_ts = TCP_SKB_CB(tcp_write_queue_head(sk))->when;
-	else
-		start_ts = tcp_sk(sk)->retrans_stamp;
+	start_ts = tcp_sk(sk)->retrans_stamp;
+	if (unlikely(!start_ts))
+		start_ts = tcp_skb_timestamp(tcp_write_queue_head(sk));
 
 	if (likely(timeout == 0)) {
 		linear_backoff_thresh = ilog2(TCP_RTO_MAX/rto_base);