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: 1d1e1de ("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>

Author: pcmoore

security/selinux/hooks.c

@@ -5778,9 +5778,9 @@ static unsigned int selinux_ip_postroute_compat(struct sk_buff *skb,
 	struct lsm_network_audit net = {0,};
 	u8 proto;
 
-	if (state->sk == NULL)
-		return NF_ACCEPT;
 	sk = skb_to_full_sk(skb);
+	if (sk == NULL)
+		return NF_ACCEPT;
 	sksec = sk->sk_security;
 
 	ad.type = LSM_AUDIT_DATA_NET;