[TCP]: Adjust Reno SACK estimate in tcp_fragment

Since the introduction of TSO pcount a year ago, it has been possible
for tcp_fragment() to cause packets_out to decrease.  Prior to that,
tcp_retrans_try_collapse() was the only way for that to happen on the
retransmission path.

When this happens with Reno, it is possible for sasked_out to become
invalid because it is only an estimate and not tied to any particular
packet on the retransmission queue.

Therefore we need to adjust sacked_out as well as left_out in the Reno
case.  The following patch does exactly that.

This bug is pretty difficult to trigger in practice though since you
need a SACKless peer with a retransmission that occurs just as the
cached MTU value expires.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -509,7 +509,16 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len, unsigned int mss
 			tp->lost_out -= diff;
 			tp->left_out -= diff;
 		}
+
 		if (diff > 0) {
+			/* Adjust Reno SACK estimate. */
+			if (!tp->rx_opt.sack_ok) {
+				tp->sacked_out -= diff;
+				if ((int)tp->sacked_out < 0)
+					tp->sacked_out = 0;
+				tcp_sync_left_out(tp);
+			}
+
 			tp->fackets_out -= diff;
 			if ((int)tp->fackets_out < 0)
 				tp->fackets_out = 0;