skbuff: add and use skb_nfct helper

Followup patch renames skb->nfct and changes its type so add a helper to
avoid intrusive rename change later.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 5a03730..cac3ebf 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -655,7 +655,7 @@ static void skb_release_head_state(struct sk_buff *skb)
 		skb->destructor(skb);
 	}
 #if IS_ENABLED(CONFIG_NF_CONNTRACK)
-	nf_conntrack_put(skb->nfct);
+	nf_conntrack_put(skb_nfct(skb));
 #endif
 #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
 	nf_bridge_put(skb->nf_bridge);