mcast: define and use MRT[6]_MAX in ip[6]_mroute_opt()

This will ease further addition of new MRT[6]_* values and avoid to update
in6.h each time.
Note that we reduce the maximum value from 210 to 209, but 210 does not match
any known value in ip[6]_mroute_setsockopt().

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/mroute6.h b/include/linux/mroute6.h
index a223561..66982e7 100644
--- a/include/linux/mroute6.h
+++ b/include/linux/mroute6.h
@@ -10,7 +10,7 @@
 #ifdef CONFIG_IPV6_MROUTE
 static inline int ip6_mroute_opt(int opt)
 {
-	return (opt >= MRT6_BASE) && (opt <= MRT6_BASE + 10);
+	return (opt >= MRT6_BASE) && (opt <= MRT6_MAX);
 }
 #else
 static inline int ip6_mroute_opt(int opt)