[TCP]: Handle SACK'd packets properly in tcp_fragment().

The problem is that we're now calling tcp_fragment() in a context
where the packets might be marked as SACKED_ACKED or SACKED_RETRANS.
This was not possible before as you never retransmitted packets that
are so marked.

Because of this, we need to adjust sacked_out and retrans_out in
tcp_fragment().  This is exactly what the following patch does.

We also need to preserve the SACKED_ACKED/SACKED_RETRANS marking
if they exist.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -461,9 +461,7 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len, unsigned int mss
 	flags = TCP_SKB_CB(skb)->flags;
 	TCP_SKB_CB(skb)->flags = flags & ~(TCPCB_FLAG_FIN|TCPCB_FLAG_PSH);
 	TCP_SKB_CB(buff)->flags = flags;
-	TCP_SKB_CB(buff)->sacked =
-		(TCP_SKB_CB(skb)->sacked &
-		 (TCPCB_LOST | TCPCB_EVER_RETRANS | TCPCB_AT_TAIL));
+	TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;
 	TCP_SKB_CB(skb)->sacked &= ~TCPCB_AT_TAIL;
 
 	if (!skb_shinfo(skb)->nr_frags && skb->ip_summed != CHECKSUM_HW) {
@@ -501,6 +499,12 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len, unsigned int mss
 			tcp_skb_pcount(buff);
 
 		tp->packets_out -= diff;
+
+		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
+			tp->sacked_out -= diff;
+		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
+			tp->retrans_out -= diff;
+
 		if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST) {
 			tp->lost_out -= diff;
 			tp->left_out -= diff;