[TCP]: Fix a bug in strategy_allowed_congestion_control

In strategy_allowed_congestion_control of the 2.6.24 kernel, when
sysctl_string return 1 on success,it should call
tcp_set_allowed_congestion_control to set the allowed congestion
control.But, it don't.  the sysctl_string return 1 on success,
otherwise return negative, never return 0.The patch fix the problem.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>