netfilter: ipv6: Preserve link scope traffic original oif

When ip6_route_me_harder is invoked, it resets outgoing interface of:
- link-local scoped packets sent by neighbor discovery
- multicast packets sent by MLD host
- multicast packets send by MLD proxy daemon that sets outgoing
interface through IPV6_PKTINFO ipi6_ifindex

Link-local and multicast packets must keep their original oif after
ip6_route_me_harder is called.

Signed-off-by: Alin Nastac <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/ipv6/netfilter.c

···
 	unsigned int hh_len;
 	struct dst_entry *dst;
 	struct flowi6 fl6 = {
-		.flowi6_oif = sk ? sk->sk_bound_dev_if : 0,
+		.flowi6_oif = sk && sk->sk_bound_dev_if ? sk->sk_bound_dev_if :
+			rt6_need_strict(&iph->daddr) ? skb_dst(skb)->dev->ifindex : 0,
 		.flowi6_mark = skb->mark,
 		.flowi6_uid = sock_net_uid(net, sk),
 		.daddr = iph->daddr,