[TCP]: Fix logic breakage due to DSACK separation

Commit 6f74651a is found guilty
of breaking DSACK counting, which should be done only for the
SACK block reported by the DSACK instead of every SACK block
that is received along with DSACK information.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -953,7 +953,7 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 	int prior_fackets;
 	u32 lost_retrans = 0;
 	int flag = 0;
-	int dup_sack = 0;
+	int found_dup_sack = 0;
 	int cached_fack_count;
 	int i;
 	int first_sack_index;
@@ -964,20 +964,20 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 
 	/* Check for D-SACK. */
 	if (before(ntohl(sp[0].start_seq), TCP_SKB_CB(ack_skb)->ack_seq)) {
-		dup_sack = 1;
+		found_dup_sack = 1;
 		tp->rx_opt.sack_ok |= 4;
 		NET_INC_STATS_BH(LINUX_MIB_TCPDSACKRECV);
 	} else if (num_sacks > 1 &&
 			!after(ntohl(sp[0].end_seq), ntohl(sp[1].end_seq)) &&
 			!before(ntohl(sp[0].start_seq), ntohl(sp[1].start_seq))) {
-		dup_sack = 1;
+		found_dup_sack = 1;
 		tp->rx_opt.sack_ok |= 4;
 		NET_INC_STATS_BH(LINUX_MIB_TCPDSACKOFORECV);
 	}
 
 	/* D-SACK for already forgotten data...
 	 * Do dumb counting. */
-	if (dup_sack &&
+	if (found_dup_sack &&
 			!after(ntohl(sp[0].end_seq), prior_snd_una) &&
 			after(ntohl(sp[0].end_seq), tp->undo_marker))
 		tp->undo_retrans--;
@@ -1058,6 +1058,7 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 		__u32 start_seq = ntohl(sp->start_seq);
 		__u32 end_seq = ntohl(sp->end_seq);
 		int fack_count;
+		int dup_sack = (found_dup_sack && (i == first_sack_index));
 
 		skb = cached_skb;
 		fack_count = cached_fack_count;