[TCP] tcp_highspeed: Fix problem observed by Xiaoliang (David) Wei

When snd_cwnd is smaller than 38 and the connection is in
congestion avoidance phase (snd_cwnd > snd_ssthresh), the snd_cwnd
seems to stop growing.

The additive increase was confused because C array's are 0 based.
Signed-off-by: NStephen Hemminger <shemminger@osdl.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_highspeed.c

@@ -135,7 +135,8 @@ static void hstcp_cong_avoid(struct sock *sk, u32 adk, u32 rtt,
 
 		/* Do additive increase */
 		if (tp->snd_cwnd < tp->snd_cwnd_clamp) {
-			tp->snd_cwnd_cnt += ca->ai;
+			/* cwnd = cwnd + a(w) / cwnd */
+			tp->snd_cwnd_cnt += ca->ai + 1;
 			if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
 				tp->snd_cwnd_cnt -= tp->snd_cwnd;
 				tp->snd_cwnd++;