bridge: netlink: add support for multicast_router

Add IFLA_BR_MCAST_ROUTER to allow setting and retrieving
br->multicast_router when igmp snooping is enabled.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 9ca9bf8..5d2c929 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -245,6 +245,7 @@
 	IFLA_BR_GC_TIMER,
 	IFLA_BR_GROUP_ADDR,
 	IFLA_BR_FDB_FLUSH,
+	IFLA_BR_MCAST_ROUTER,
 	__IFLA_BR_MAX,
 };