xfrm: remove afinfo pointer from xfrm_mode

Adds an EXPORT_SYMBOL for afinfo_get_rcu, as it will now be called from
ipv6 in case of CONFIG_IPV6=m.

This change has virtually no effect on vmlinux size, but it reduces
afinfo size and allows followup patch to make xfrm modes const.

v2: mark if (afinfo) tests as likely (Sabrina)
    re-fetch afinfo according to inner_mode in xfrm_prepare_input().

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/xfrm4_output.c

@@ -72,6 +72,8 @@ int xfrm4_output_finish(struct sock *sk, struct sk_buff *skb)
 static int __xfrm4_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
 	struct xfrm_state *x = skb_dst(skb)->xfrm;
+	const struct xfrm_state_afinfo *afinfo;
+	int ret = -EAFNOSUPPORT;
 
 #ifdef CONFIG_NETFILTER
 	if (!x) {
@@ -80,7 +82,15 @@ static int __xfrm4_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 	}
 #endif
 
-	return x->outer_mode->afinfo->output_finish(sk, skb);
+	rcu_read_lock();
+	afinfo = xfrm_state_afinfo_get_rcu(x->outer_mode->family);
+	if (likely(afinfo))
+		ret = afinfo->output_finish(sk, skb);
+	else
+		kfree_skb(skb);
+	rcu_read_unlock();
+
+	return ret;
 }
 
 int xfrm4_output(struct net *net, struct sock *sk, struct sk_buff *skb)