netfilter: xt_TEE: don't use destination address found in header

Torsten Luettgert bisected TEE regression starting with commit
f8126f1d (ipv4: Adjust semantics of rt->rt_gateway.)

The problem is that it tries to ARP-lookup the original destination
address of the forwarded packet, not the address of the gateway.

Fix this using FLOWI_FLAG_KNOWN_NH Julian added in commit
c92b9655 (ipv4: Add FLOWI_FLAG_KNOWN_NH), so that known
nexthop (info->gw.ip) has preference on resolving.
Reported-by: Torsten Luettgert <ml-netfilter@enda.eu>
Bisected-by: Torsten Luettgert <ml-netfilter@enda.eu>
Tested-by: Torsten Luettgert <ml-netfilter@enda.eu>
Cc: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_TEE.c

@@ -70,6 +70,7 @@ tee_tg_route4(struct sk_buff *skb, const struct xt_tee_tginfo *info)
 	fl4.daddr = info->gw.ip;
 	fl4.flowi4_tos = RT_TOS(iph->tos);
 	fl4.flowi4_scope = RT_SCOPE_UNIVERSE;
+	fl4.flowi4_flags = FLOWI_FLAG_KNOWN_NH;
 	rt = ip_route_output_key(net, &fl4);
 	if (IS_ERR(rt))
 		return false;