tcp: Fix tcp_hybla zero congestion window growth with small rho and large cwnd.

Because of rounding, in certain conditions, i.e. when in congestion
avoidance state rho is smaller than 1/128 of the current cwnd, TCP
Hybla congestion control starves and the cwnd is kept constant
forever.

This patch forces an increment by one segment after #send_cwnd calls
without increments(newreno behavior).
Signed-off-by: NDaniele Lacamera <root@danielinux.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_hybla.c

@@ -150,7 +150,11 @@ static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 		ca->snd_cwnd_cents -= 128;
 		tp->snd_cwnd_cnt = 0;
 	}
-
+	/* check when cwnd has not been incremented for a while */
+	if (increment == 0 && odd == 0 && tp->snd_cwnd_cnt >= tp->snd_cwnd) {
+		tp->snd_cwnd++;
+		tp->snd_cwnd_cnt = 0;
+	}
 	/* clamp down slowstart cwnd to ssthresh value. */
 	if (is_slowstart)
 		tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);