xfrm: choose protocol family by skb protocol

We need to choose the protocol family by skb->protocol. Otherwise we
call the wrong xfrm{4,6}_local_error handler in case an ipv6 sockets is
used in ipv4 mode, in which case we should call down to xfrm4_local_error
(ip6 sockets are a superset of ip4 ones).

We are called before before ip_output functions, so skb->protocol is
not reset.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/net/xfrm/xfrm_output.c b/net/xfrm/xfrm_output.c
index 6f5fc61..3bb2cdc 100644
--- a/net/xfrm/xfrm_output.c
+++ b/net/xfrm/xfrm_output.c
@@ -216,9 +216,17 @@
 
 void xfrm_local_error(struct sk_buff *skb, int mtu)
 {
+	unsigned int proto;
 	struct xfrm_state_afinfo *afinfo;
 
-	afinfo = xfrm_state_get_afinfo(skb->sk->sk_family);
+	if (skb->protocol == htons(ETH_P_IP))
+		proto = AF_INET;
+	else if (skb->protocol == htons(ETH_P_IPV6))
+		proto = AF_INET6;
+	else
+		return;
+
+	afinfo = xfrm_state_get_afinfo(proto);
 	if (!afinfo)
 		return;