net: dsa: only bring down user ports assigned to a given DSA master

This is an adaptation of commit c0a8a9c ("net: dsa: automatically
bring user ports down when master goes down") for multiple DSA masters.
When a DSA master goes down, only the user ports under its control
should go down too, the others can still send/receive traffic.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/dsa/slave.c

@@ -2873,6 +2873,9 @@ static int dsa_slave_netdevice_event(struct notifier_block *nb,
 			if (!dsa_port_is_user(dp))
 				continue;
 
+			if (dp->cpu_dp != cpu_dp)
+				continue;
+
 			list_add(&dp->slave->close_list, &close_list);
 		}