ipv4: Set rt->rt_iif more sanely on output routes.

rt->rt_iif is only ever inspected on input routes, for example DCCP
uses this to populate a route lookup flow key when generating replies
to another packet.

Therefore, setting it to anything other than zero on output routes
makes no sense.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -2381,7 +2381,7 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
 	rth->fl.mark    = oldflp->mark;
 	rth->rt_dst	= fl->fl4_dst;
 	rth->rt_src	= fl->fl4_src;
-	rth->rt_iif	= oldflp->oif ? : dev_out->ifindex;
+	rth->rt_iif	= 0;
 	/* get references to the devices that are to be hold by the routing
 	   cache entry */
 	rth->dst.dev	= dev_out;