net: #ifdefify sk_classid member of struct sock

The sk_classid member is only required when CONFIG_CGROUP_NET_CLASSID is
enabled. #ifdefify it to reduce the size of struct sock on 32 bit
systems, at least.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>