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/include/linux/skbuff.h b/include/linux/skbuff.h
index b53c0cf..276431e 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3553,6 +3553,15 @@ static inline void skb_remcsum_process(struct sk_buff *skb, void *ptr,
 	skb->csum = csum_add(skb->csum, delta);
 }
 
+static inline struct nf_conntrack *skb_nfct(const struct sk_buff *skb)
+{
+#if IS_ENABLED(CONFIG_NF_CONNTRACK)
+	return skb->nfct;
+#else
+	return NULL;
+#endif
+}
+
 #if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
 void nf_conntrack_destroy(struct nf_conntrack *nfct);
 static inline void nf_conntrack_put(struct nf_conntrack *nfct)
@@ -3652,9 +3661,7 @@ static inline bool skb_irq_freeable(const struct sk_buff *skb)
 #if IS_ENABLED(CONFIG_XFRM)
 		!skb->sp &&
 #endif
-#if IS_ENABLED(CONFIG_NF_CONNTRACK)
-		!skb->nfct &&
-#endif
+		!skb_nfct(skb) &&
 		!skb->_skb_refdst &&
 		!skb_has_frag_list(skb);
 }