xfrm: Decode sessions with output interface.

The output interface matching does not work on forward
policy lookups, the output interface of the flowi is
always 0. Fix this by setting the output interface when
we decode the session.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index 9a459be..ccde542 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -107,6 +107,7 @@
 
 	memset(fl4, 0, sizeof(struct flowi4));
 	fl4->flowi4_mark = skb->mark;
+	fl4->flowi4_oif = skb_dst(skb)->dev->ifindex;
 
 	if (!ip_is_fragment(iph)) {
 		switch (iph->protocol) {