tcp: remove ack_time from struct tcp_sacktag_state

It is no longer needed, everything uses tp->tcp_mstamp instead.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NSoheil Hassas Yeganeh <soheil@google.com>
Acked-by: NNeal Cardwell <ncardwell@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1131,7 +1131,6 @@ struct tcp_sacktag_state {
 	 */
 	struct skb_mstamp first_sackt;
 	struct skb_mstamp last_sackt;
-	struct skb_mstamp ack_time; /* Timestamp when the S/ACK was received */
 	struct rate_sample *rate;
 	int	flag;
 };
@@ -3572,8 +3571,6 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	if (after(ack, tp->snd_nxt))
 		goto invalid_ack;
 
-	skb_mstamp_get(&sack_state.ack_time);
-
 	if (icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
 		tcp_rearm_rto(sk);
 
@@ -3684,7 +3681,6 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	 * If data was DSACKed, see if we can undo a cwnd reduction.
 	 */
 	if (TCP_SKB_CB(skb)->sacked) {
-		skb_mstamp_get(&sack_state.ack_time);
 		flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
 						&sack_state);
 		tcp_fastretrans_alert(sk, acked, is_dupack, &flag, &rexmit);