ipv4: Make rt->fl.iif tests lest obscure.

When we test rt->fl.iif against zero, we're seeing if it's
an output or an input route.

Make that explicit with some helper functions.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index c8877c6..08d0d81 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -961,7 +961,7 @@
 	case IGMP_HOST_MEMBERSHIP_REPORT:
 	case IGMPV2_HOST_MEMBERSHIP_REPORT:
 		/* Is it our report looped back? */
-		if (skb_rtable(skb)->fl.iif == 0)
+		if (rt_is_output_route(skb_rtable(skb)))
 			break;
 		/* don't rely on MC router hearing unicast reports */
 		if (skb->pkt_type == PACKET_MULTICAST ||