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/ip_gre.c b/net/ipv4/ip_gre.c
index 70ff77f..cab2057 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -634,7 +634,7 @@
 #ifdef CONFIG_NET_IPGRE_BROADCAST
 		if (ipv4_is_multicast(iph->daddr)) {
 			/* Looped back packet, drop it! */
-			if (skb_rtable(skb)->fl.iif == 0)
+			if (rt_is_output_route(skb_rtable(skb)))
 				goto drop;
 			tunnel->dev->stats.multicast++;
 			skb->pkt_type = PACKET_BROADCAST;