net: dsa: add port fast ageing

Today the DSA drivers are in charge of flushing the MAC addresses
associated to a port when its STP state changes from Learning or
Forwarding, to Disabled or Blocking or Listening.

This makes the drivers more complex and hides the generic switch logic.
Introduce a new optional port_fast_age operation to dsa_switch_ops, to
move this logic to the DSA layer and keep drivers simple.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>