mlxsw: spectrum_switchdev: Replay switchdev objects on port join

Currently it never happens that a netdevice that is already a bridge slave
would suddenly become mlxsw upper. The only case where this might be
possible as far as mlxsw is concerned, is with LAG netdevices. But if a LAG
has any upper (e.g. is enslaved), enlaving mlxsw port to that LAG is
forbidden. Thus the only way to install a LAG between a bridge and a mlxsw
port is by first enslaving the port to the LAG, and then enslaving that LAG
to a bridge. At that point there are no bridge objects (such as port VLANs)
to replay. Those are added afterwards, and notified as they are created.
This holds even for the PVID.

However in the following patches, the requirement that ports be only
enslaved to masters without uppers, is going to be relaxed. It will
therefore be necessary to replay the existing bridge objects. Without this
replay, e.g. the mlxsw bridge_port_vlan objects are not instantiated, which
causes issues later, as a lot of code relies on their presence.

To that end, add a new notifier block whose sole role is to filter out
events related to the one relevant upper, and forward those to the existing
switchdev notifier block. Pass the new notifier block to
switchdev_bridge_port_offload() when the bridge port is created.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>