[TCP]: Don't enter to fast recovery while using FRTO

Because TCP is not in Loss state during FRTO recovery, fast
recovery could be triggered by accident. Non-SACK FRTO is more
robust than not yet included SACK-enhanced version (that can
receiver high number of duplicate ACKs with SACK blocks during
FRTO), at least with unidirectional transfers, but under
extraordinary patterns fast recovery can be incorrectly
triggered, e.g., Data loss+ACK losses => cumulative ACK with
enough SACK blocks to meet sacked_out >= dupthresh condition).
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1547,6 +1547,10 @@ static int tcp_time_to_recover(struct sock *sk, struct tcp_sock *tp)
 {
 	__u32 packets_out;
 
+	/* Do not perform any recovery during FRTO algorithm */
+	if (tp->frto_counter)
+		return 0;
+
 	/* Trick#1: The loss is proven. */
 	if (tp->lost_out)
 		return 1;