[TCP] illinois: Incorrect beta usage

Lachlan Andrew observed that my TCP-Illinois implementation uses the
beta value incorrectly:
  The parameter  beta  in the paper specifies the amount to decrease
  *by*:  that is, on loss,
     W <-  W -  beta*W
  but in   tcp_illinois_ssthresh() uses  beta  as the amount
  to decrease  *to*: W <- beta*W

This bug makes the Linux TCP-Illinois get less-aggressive on uncongested network,
hurting performance. Note: since the base beta value is .5, it has no
impact on a congested network.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

net/ipv4/tcp_illinois.c

@@ -298,7 +298,7 @@ static u32 tcp_illinois_ssthresh(struct sock *sk)
 	struct illinois *ca = inet_csk_ca(sk);
 
 	/* Multiplicative decrease */
-	return max((tp->snd_cwnd * ca->beta) >> BETA_SHIFT, 2U);
+	return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->beta) >> BETA_SHIFT), 2U);
 }