[TCP]: Consider GSO while counting reord in sacktag

Reordering detection fails to take account that the reordered
skb may have pcount larger than 1. In such case the lowest of
them had the largest reordering, the old formula used the
highest of them which is pcount - 1 packets less reordered.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1403,8 +1403,6 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 			if (in_sack < 0)
 				break;
 
-			fack_count += tcp_skb_pcount(skb);
-
 			sacked = TCP_SKB_CB(skb)->sacked;
 
 			/* Account D-SACK for retransmitted packet. */
@@ -1427,11 +1425,14 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 				}
 
 				/* Nothing to do; acked frame is about to be dropped. */
+				fack_count += tcp_skb_pcount(skb);
 				continue;
 			}
 
-			if (!in_sack)
+			if (!in_sack) {
+				fack_count += tcp_skb_pcount(skb);
 				continue;
+			}
 
 			if (!(sacked&TCPCB_SACKED_ACKED)) {
 				if (sacked & TCPCB_SACKED_RETRANS) {
@@ -1480,6 +1481,7 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 				flag |= FLAG_DATA_SACKED;
 				tp->sacked_out += tcp_skb_pcount(skb);
 
+				fack_count += tcp_skb_pcount(skb);
 				if (fack_count > tp->fackets_out)
 					tp->fackets_out = fack_count;
 
@@ -1490,6 +1492,8 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 			} else {
 				if (dup_sack && (sacked&TCPCB_RETRANS))
 					reord = min(fack_count, reord);
+
+				fack_count += tcp_skb_pcount(skb);
 			}
 
 			/* D-SACK. We can detect redundant retransmission
@@ -1515,7 +1519,7 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 
 	if ((reord < tp->fackets_out) && icsk->icsk_ca_state != TCP_CA_Loss &&
 	    (!tp->frto_highmark || after(tp->snd_una, tp->frto_highmark)))
-		tcp_update_reordering(sk, ((tp->fackets_out + 1) - reord), 0);
+		tcp_update_reordering(sk, tp->fackets_out - reord, 0);
 
 #if FASTRETRANS_DEBUG > 0
 	BUG_TRAP((int)tp->sacked_out >= 0);