tcp: add in_flight to tcp_skb_cb

Add in_flight (bytes in flight when packet was sent) field
to tx component of tcp_skb_cb and make it available to
congestion modules' pkts_acked() function through the
ack_sample function argument.

Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 8bd9911..b1bcba0 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -911,9 +911,12 @@
 	int err;
 
 	BUG_ON(!skb || !tcp_skb_pcount(skb));
+	tp = tcp_sk(sk);
 
 	if (clone_it) {
 		skb_mstamp_get(&skb->skb_mstamp);
+		TCP_SKB_CB(skb)->tx.in_flight = TCP_SKB_CB(skb)->end_seq
+			- tp->snd_una;
 
 		if (unlikely(skb_cloned(skb)))
 			skb = pskb_copy(skb, gfp_mask);
@@ -924,7 +927,6 @@
 	}
 
 	inet = inet_sk(sk);
-	tp = tcp_sk(sk);
 	tcb = TCP_SKB_CB(skb);
 	memset(&opts, 0, sizeof(opts));