net: expose devlink port over rtnetlink

Expose devlink port handle related to netdev over rtnetlink. Introduce a
new nested IFLA attribute to carry the info. Call into devlink code to
fill-up the nest with existing devlink attributes that are used over
devlink netlink.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>