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/decnet/dn_route.c b/net/decnet/dn_route.c
index 94a9eb1..474d54d 100644
--- a/net/decnet/dn_route.c
+++ b/net/decnet/dn_route.c
@@ -1181,7 +1181,7 @@
 			if ((flp->fld_dst == rt->fl.fld_dst) &&
 			    (flp->fld_src == rt->fl.fld_src) &&
 			    (flp->mark == rt->fl.mark) &&
-			    (rt->fl.iif == 0) &&
+			    dn_is_output_route(rt) &&
 			    (rt->fl.oif == flp->oif)) {
 				dst_use(&rt->dst, jiffies);
 				rcu_read_unlock_bh();
@@ -1512,7 +1512,7 @@
 	if (rtnl_put_cacheinfo(skb, &rt->dst, 0, 0, 0, expires,
 			       rt->dst.error) < 0)
 		goto rtattr_failure;
-	if (rt->fl.iif)
+	if (dn_is_input_route(rt))
 		RTA_PUT(skb, RTA_IIF, sizeof(int), &rt->fl.iif);
 
 	nlh->nlmsg_len = skb_tail_pointer(skb) - b;