netfilter: af_info: add 'strict' parameter to limit lookup to .oif

ipv6 fib lookup can set RT6_LOOKUP_F_IFACE flag to restrict search
to an interface, but this flag cannot be set via struct flowi.

Also, it cannot be set via ip6_route_output: this function uses the
passed sock struct to determine if this flag is required
(by testing for nonzero sk_bound_dev_if).

Work around this by passing in an artificial struct sk in case
'strict' argument is true.

This is required to replace the rt6_lookup call in xt_addrtype.c with
nf_afinfo->route().

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/include/linux/netfilter.h b/include/linux/netfilter.h
index 20ed452..7fa95df 100644
--- a/include/linux/netfilter.h
+++ b/include/linux/netfilter.h
@@ -271,7 +271,7 @@
 					    unsigned int len,
 					    u_int8_t protocol);
 	int		(*route)(struct net *net, struct dst_entry **dst,
-				 struct flowi *fl);
+				 struct flowi *fl, bool strict);
 	void		(*saveroute)(const struct sk_buff *skb,
 				     struct nf_queue_entry *entry);
 	int		(*reroute)(struct sk_buff *skb,