netlink: add RTA_MAX definition
#define RTA_OIF 4 /* Argument: Output interface index */
#define RTA_GATEWAY 5 /* Argument: Gateway address of the route */
#define RTA_GENMASK 6 /* Argument: Network address mask of sub-net */
+#define RTA_MAX 6 /* MAX type, same as last argument */
/* NETLINK_ROUTE protocol message types *************************************/
