[TCP]: Move prior_in_flight collect to more robust place

The previous location is after sacktag processing, which affects
counters tcp_packets_in_flight depends on. This may manifest as
wrong behavior if new SACK blocks are present and all is clear
for call to tcp_cong_avoid, which in the case of
tcp_reno_cong_avoid bails out early because it thinks that
TCP is not limited by cwnd.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -3065,6 +3065,7 @@ static int tcp_ack(struct sock *sk, struct sk_buff *skb, int flag)
 	}
 
 	prior_fackets = tp->fackets_out;
+	prior_in_flight = tcp_packets_in_flight(tp);
 
 	if (!(flag&FLAG_SLOWPATH) && after(ack, prior_snd_una)) {
 		/* Window is constant, pure forward advance.
@@ -3104,8 +3105,6 @@ static int tcp_ack(struct sock *sk, struct sk_buff *skb, int flag)
 	if (!prior_packets)
 		goto no_queue;
 
-	prior_in_flight = tcp_packets_in_flight(tp);
-
 	/* See if we can take anything off of the retransmit queue. */
 	flag |= tcp_clean_rtx_queue(sk, &seq_rtt, prior_fackets);