tcp: fix TSO FACK loss marking in tcp_mark_head_lost

When TCP uses FACK algorithm to mark lost packets in
tcp_mark_head_lost(), if the number of packets in the (TSO) skb is
greater than the number of packets that should be marked lost, TCP
incorrectly exits the loop and marks no packets lost in the skb. This
underestimates tp->lost_out and affects the recovery/retransmission.
This patch fargments the skb and marks the correct amount of packets
lost.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -2545,7 +2545,8 @@ static void tcp_mark_head_lost(struct sock *sk, int packets)
 			cnt += tcp_skb_pcount(skb);
 
 		if (cnt > packets) {
-			if (tcp_is_sack(tp) || (oldcnt >= packets))
+			if ((tcp_is_sack(tp) && !tcp_is_fack(tp)) ||
+			    (oldcnt >= packets))
 				break;
 
 			mss = skb_shinfo(skb)->gso_size;