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/icmp.c b/net/ipv4/icmp.c
index 96bc7f9..c6e2aff 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -506,8 +506,8 @@
 		struct net_device *dev = NULL;
 
 		rcu_read_lock();
-		if (rt->fl.iif &&
-			net->ipv4.sysctl_icmp_errors_use_inbound_ifaddr)
+		if (rt_is_input_route(rt) &&
+		    net->ipv4.sysctl_icmp_errors_use_inbound_ifaddr)
 			dev = dev_get_by_index_rcu(net, rt->fl.iif);
 
 		if (dev)