bridge: netlink: export root id

Add IFLA_BR_ROOT_ID and export br->designated_root via netlink. For this
purpose add struct ifla_bridge_id that would represent struct bridge_id.

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 f7f4806..0381437 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -233,11 +233,17 @@
 	IFLA_BR_VLAN_FILTERING,
 	IFLA_BR_VLAN_PROTOCOL,
 	IFLA_BR_GROUP_FWD_MASK,
+	IFLA_BR_ROOT_ID,
 	__IFLA_BR_MAX,
 };
 
 #define IFLA_BR_MAX	(__IFLA_BR_MAX - 1)
 
+struct ifla_bridge_id {
+	__u8	prio[2];
+	__u8	addr[6]; /* ETH_ALEN */
+};
+
 enum {
 	BRIDGE_MODE_UNSPEC,
 	BRIDGE_MODE_HAIRPIN,