sock->sk_cgrp_prioidx won't be used at all if CONFIG_NETPRIO_CGROUP=n.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>