mlxsw: spectrum: Refactor FDB flushing logic

FDB entries are learned using {Port / LAG ID, FID} and therefore should
be flushed whenever a port (vPort) leaves its FID (vFID).

However, when the bridge port is a LAG device (or a VLAN device on top),
then FDB flushing is conditional. Ports removed from such LAG
configurations must not trigger flushing, as other ports might still be
members in the LAG and therefore the bridge port is still active.

The decision whether to flush or not was previously computed in the
netdevice notification block, but in order to flush the entries when a
port leaves its FID this decision should be computed there.

Strip the notification block from this logic and instead move it to one
FDB flushing function that is invoked from both the FID / vFID leave
functions.

When port isn't member in LAG, FDB flushing should always occur.
Otherwise, it should occur only when the last port (vPort) member in the
LAG leaves the FID (vFID).

This will allow us - in the next patch - to simplify the cleanup code
paths that are hit whenever the topology above the port netdevs changes.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>