mlxsw: spectrum: Move netdevice NB to struct mlxsw_sp

mlxsw: spectrum: Move netdevice NB to struct mlxsw_sp

So far, all netdevice notifications that the driver cared about were
related to its own ports, and mlxsw_sp could be retrieved from the
netdevice's private data. For IP-in-IP offloading however, the driver
cares about events on foreign netdevices, and getting at mlxsw_sp or
router data structures from the handler is inconvenient.

Therefore move the netdevice notifier blocks from global scope to struct
mlxsw_sp to allow retrieval from the notifier block pointer itself.

Signed-off-by: Petr Machata <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Signed-off-by: Jiri Pirko <[email protected]>
Signed-off-by: David S. Miller <[email protected]>