tcp: use DSACKs that arrive when packets_out is 0

The bug: Senders ignored DSACKs after recovery when there were no
outstanding packets (a common scenario for HTTP servers).

The change: when there are no outstanding packets (the "no_queue" goto
label), call tcp_fastretrans_alert() in order to use DSACKs to undo
congestion window reductions.

Other patches in this series will provide other changes that are
necessary to fully fix this problem.

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

net/ipv4/tcp_input.c

@@ -3788,6 +3788,10 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	return 1;
 
 no_queue:
+	/* If data was DSACKed, see if we can undo a cwnd reduction. */
+	if (flag & FLAG_DSACKING_ACK)
+		tcp_fastretrans_alert(sk, pkts_acked, newly_acked_sacked,
+				      is_dupack, flag);
 	/* If this ack opens up a zero window, clear backoff.  It was
 	 * being used to time the probes, and is probably far higher than
 	 * it needs to be for normal retransmission.