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/ipmr.c b/net/ipv4/ipmr.c
index 86dd569..ef2b008 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -1654,7 +1654,7 @@
 	if (mrt->vif_table[vif].dev != skb->dev) {
 		int true_vifi;
 
-		if (skb_rtable(skb)->fl.iif == 0) {
+		if (rt_is_output_route(skb_rtable(skb))) {
 			/* It is our own packet, looped back.
 			 * Very complicated situation...
 			 *