net: dsa: don't fast age bridge ports with learning turned off

On topology changes, stations that were dynamically learned on ports
that are no longer part of the active topology must be flushed - this is
described by clause "17.11 Updating learned station location information"
of IEEE 802.1D-2004.

However, when address learning on the bridge port is turned off in the
first place, there is nothing to flush, so skip a potentially expensive
operation.

We can finally do this now since DSA is aware of the learning state of
its bridged ports.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/port.c

@@ -50,7 +50,7 @@ int dsa_port_set_state(struct dsa_port *dp, u8 state, bool do_fast_age)
 
 	ds->ops->port_stp_state_set(ds, port, state);
 
-	if (do_fast_age) {
+	if (do_fast_age && dp->learning) {
 		/* Fast age FDB entries or flush appropriate forwarding database
 		 * for the given port, if we are moving it from Learning or
 		 * Forwarding state, to Disabled or Blocking or Listening state.