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>