ipv4: Turn rt->rt_route_iif into rt->rt_is_input.

That is this value's only use, as a boolean to indicate whether
a route is an input route or not.

So implement it that way, using a u16 gap present in the struct
already.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/route.h b/include/net/route.h
index e789a92..4bafe0b 100644
--- a/include/net/route.h
+++ b/include/net/route.h
@@ -47,8 +47,8 @@
 	int			rt_genid;
 	unsigned int		rt_flags;
 	__u16			rt_type;
+	__u16			rt_is_input;
 
-	int			rt_route_iif;
 	int			rt_iif;
 
 	/* Info on neighbour */
@@ -61,12 +61,12 @@
 
 static inline bool rt_is_input_route(const struct rtable *rt)
 {
-	return rt->rt_route_iif != 0;
+	return rt->rt_is_input != 0;
 }
 
 static inline bool rt_is_output_route(const struct rtable *rt)
 {
-	return rt->rt_route_iif == 0;
+	return rt->rt_is_input == 0;
 }
 
 static inline __be32 rt_nexthop(const struct rtable *rt, __be32 daddr)