[TCP] illinois: Incorrect beta usage

[ Upstream commit: a357dde9 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>