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>

include/net/dn_route.h

@@ -80,6 +80,16 @@ struct dn_route {
 	unsigned rt_type;
 };
 
+static inline bool dn_is_input_route(struct dn_route *rt)
+{
+	return rt->fl.iif != 0;
+}
+
+static inline bool dn_is_output_route(struct dn_route *rt)
+{
+	return rt->fl.iif == 0;
+}
+
 extern void dn_route_init(void);
 extern void dn_route_cleanup(void);