[TCP]: Prevent reordering adjustments during FRTO

To be honest, I'm not too sure how the reord stuff works in the
first place but this seems necessary.

When FRTO has been active, the one and only retransmission could
be unnecessary but the state and sending order might not be what
the sacktag code expects it to be (to work correctly).
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1224,7 +1224,8 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 
 	tp->left_out = tp->sacked_out + tp->lost_out;
 
-	if ((reord < tp->fackets_out) && icsk->icsk_ca_state != TCP_CA_Loss)
+	if ((reord < tp->fackets_out) && icsk->icsk_ca_state != TCP_CA_Loss &&
+	    (tp->frto_highmark && after(tp->snd_una, tp->frto_highmark)))
 		tcp_update_reordering(sk, ((tp->fackets_out + 1) - reord), 0);
 
 #if FASTRETRANS_DEBUG > 0