[TCP] FRTO: Limit snd_cwnd if TCP was application limited

Otherwise TCP might violate packet ordering principles that FRTO
is based on. If conventional recovery path is chosen, this won't
be significant at all. In practice, any small enough value will
be sufficient to provide proper operation for FRTO, yet other
users of snd_cwnd might benefit from a "close enough" value.

FRTO's formula is now equal to what tcp_enter_cwr() uses.

FRTO used to check application limitedness a bit differently but
I changed that in commit 575ee714
and as a result checking for application limitedness became
completely non-existing.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1669,6 +1669,9 @@ void tcp_enter_frto(struct sock *sk)
 	}
 	tcp_verify_left_out(tp);
 
+	/* Too bad if TCP was application limited */
+	tp->snd_cwnd = min(tp->snd_cwnd, tcp_packets_in_flight(tp) + 1);
+
 	/* Earlier loss recovery underway (see RFC4138; Appendix B).
 	 * The last condition is necessary at least in tp->frto_counter case.
 	 */