tcp: tcp_rack_reo_timeout() must update tp->tcp_mstamp

I wrongly assumed tp->tcp_mstamp was up to date at the time
tcp_rack_reo_timeout() was called.

It is not true, since we only update tcp->tcp_mstamp when receiving
a packet (as initially done in commit 69e996c58a ("tcp: add
tp->tcp_mstamp field")

tcp_rack_reo_timeout() being called by a timer and not an incoming
packet, we need to refresh tp->tcp_mstamp

Fixes: 7c1c730859 ("tcp: do not pass timestamp to tcp_rack_detect_loss()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_recovery.c

@@ -166,6 +166,7 @@ void tcp_rack_reo_timeout(struct sock *sk)
 	u32 timeout, prior_inflight;
 
 	prior_inflight = tcp_packets_in_flight(tp);
+	skb_mstamp_get(&tp->tcp_mstamp);
 	tcp_rack_detect_loss(sk, &timeout);
 	if (prior_inflight != tcp_packets_in_flight(tp)) {
 		if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {