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: NSteffen Klassert <steffen.klassert@secunet.com>

net/ipv4/xfrm4_policy.c

@@ -107,6 +107,7 @@ _decode_session4(struct sk_buff *skb, struct flowi *fl, int reverse)
 
 	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) {

net/ipv6/xfrm6_policy.c

@@ -138,6 +138,7 @@ _decode_session6(struct sk_buff *skb, struct flowi *fl, int reverse)
 
 	memset(fl6, 0, sizeof(struct flowi6));
 	fl6->flowi6_mark = skb->mark;
+	fl6->flowi6_oif = skb_dst(skb)->dev->ifindex;
 
 	fl6->daddr = reverse ? hdr->saddr : hdr->daddr;
 	fl6->saddr = reverse ? hdr->daddr : hdr->saddr;