rocker: Handle SWITCHDEV_PORT_OBJ_ADD/_DEL

Following patches will change the way of distributing port object
changes from a switchdev operation to a switchdev notifier. The
switchdev code currently recursively descends through layers of lower
devices, eventually calling the op on a front-panel port device. The
notifier will instead be sent referencing the bridge port device, which
may be a stacking device that's one of front-panel ports uppers, or a
completely unrelated device.

rocker currently doesn't support any uppers other than bridge. Thus the
only case that a stacked device could be validly referenced by port
object notifications are bridge notifications for VLAN objects added to
the bridge itself. But the driver explicitly rejects such notifications
in rocker_world_port_obj_vlan_add(). It is therefore safe to assume that
the only interesting case is that the notification is on a front-panel
port netdevice.

Subscribe to the blocking notifier chain. In the handler, filter out
notifications on any foreign netdevices. Dispatch the new notifiers to
rocker_port_obj_add() resp. _del() to maintain the behavior that the
switchdev operation based code currently has.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>