devlink: create a special NDO for getting the devlink instance

Instead of iterating over all devlink ports add a NDO which
will return the devlink instance from the driver.

v2: add the netdev_to_devlink() helper (Michal)
v3: check that devlink has ops (Florian)
v4: hold devlink_mutex (Jiri)

Suggested-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>