bridge: implement rtnl_link_ops->get_slave_size and rtnl_link_ops->fill_slave_info

Allow rtnetlink users to get port info in IFLA_INFO_SLAVE_DATA attr

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>