bridge: del external_learned fdbs from device on flush or ageout

We need to delete from offload the device externally learnded fdbs when any
one of these events happen:

1) Bridge ages out fdb.  (When bridge is doing ageing vs. device doing
ageing.  If device is doing ageing, it would send SWITCHDEV_FDB_DEL
directly).

2) STP state change flushes fdbs on port.

3) User uses sysfs interface to flush fdbs from bridge or bridge port:

	echo 1 >/sys/class/net/BR_DEV/bridge/flush
	echo 1 >/sys/class/net/BR_PORT/brport/flush

4) Offload driver send event SWITCHDEV_FDB_DEL to delete fdb entry.

For rocker, we can now get called to delete fdb entry in wait and nowait
contexts, so set NOWAIT flag when deleting fdb entry.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>