selinux: use a consistent method to get full socket from skb

In order to maintain code consistency and readability,
skb_to_full_sk() is used to get full socket from skb.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>