[NETFILTER]: nf_nat: properly use RCU for ip_nat_decode_session

We need to use rcu_assign_pointer/rcu_dereference to avoid races.
Also remove an obsolete CONFIG_IP_NAT_NEEDED ifdef.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netfilter.h b/include/linux/netfilter.h
index 0947424..1a84873 100644
--- a/include/linux/netfilter.h
+++ b/include/linux/netfilter.h
@@ -256,11 +256,16 @@
 static inline void
 nf_nat_decode_session(struct sk_buff *skb, struct flowi *fl, int family)
 {
-#if defined(CONFIG_IP_NF_NAT_NEEDED) || defined(CONFIG_NF_NAT_NEEDED)
+#ifdef CONFIG_NF_NAT_NEEDED
 	void (*decodefn)(struct sk_buff *, struct flowi *);
 
-	if (family == AF_INET && (decodefn = ip_nat_decode_session) != NULL)
-		decodefn(skb, fl);
+	if (family == AF_INET) {
+		rcu_read_lock();
+		decodefn = rcu_dereference(ip_nat_decode_session);
+		if (decodefn)
+			decodefn(skb, fl);
+		rcu_read_unlock();
+	}
 #endif
 }