bridge: netlink: export port's root id

Add IFLA_BRPORT_ROOT_ID to allow getting the designated root id via
netlink.

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 c5b15bf..c45c1d7 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -292,6 +292,7 @@
 	IFLA_BRPORT_PROXYARP,	/* proxy ARP */
 	IFLA_BRPORT_LEARNING_SYNC, /* mac learning sync from device */
 	IFLA_BRPORT_PROXYARP_WIFI, /* proxy ARP for Wi-Fi */
+	IFLA_BRPORT_ROOT_ID,	/* designated root */
 	__IFLA_BRPORT_MAX
 };
 #define IFLA_BRPORT_MAX (__IFLA_BRPORT_MAX - 1)