ipv4: Restore flowi4_oif update before call to xfrm_lookup_route

Tobias reported regressions in IPsec tests following the patch
referenced by the Fixes tag below. The root cause is dropping the
reset of the flowi4_oif after the fib_lookup. Apparently it is
needed for xfrm cases, so restore the oif update to ip_route_output_flow
right before the call to xfrm_lookup_route.

Fixes: 2fbc6e89b2f1 ("ipv4: Update exception handling for multipath routes via same device")
Reported-by: Tobias Brunner <[e-mail]>
Signed-off-by: David Ahern <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv4/route.c

···
 	if (IS_ERR(rt))
 		return rt;
 
-	if (flp4->flowi4_proto)
+	if (flp4->flowi4_proto) {
+		flp4->flowi4_oif = rt->dst.dev->ifindex;
 		rt = (struct rtable *)xfrm_lookup_route(net, &rt->dst,
 							flowi4_to_flowi(flp4),
 							sk, 0);
+	}
 
 	return rt;
 }