selinux: fix a sock regression in selinux_ip_postroute_compat()

Unfortunately we can't rely on nf_hook_state->sk being the proper
originating socket so revert to using skb_to_full_sk(skb).

Fixes: 1d1e1ded ("selinux: make better use of the nf_hook_state passed to the NF hooks")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Paul Moore <paul@paul-moore.com>