net: bridge: optimize backup_port fdb convergence

We can optimize the fdb convergence when a backup_port is present by not
immediately flushing the entries of the stopped port since traffic for
those entries will flow towards the backup_port.

There are 2 cases specifically that benefit most:
- when the stopped port comes up before the entries expire by themselves
- when there's an external entry refresh and they're kept while the
  backup_port is operating (e.g. mlag)

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_stp_if.c

@@ -117,7 +117,8 @@ void br_stp_disable_port(struct net_bridge_port *p)
 	del_timer(&p->forward_delay_timer);
 	del_timer(&p->hold_timer);
 
-	br_fdb_delete_by_port(br, p, 0, 0);
+	if (!rcu_access_pointer(p->backup_port))
+		br_fdb_delete_by_port(br, p, 0, 0);
 	br_multicast_disable_port(p);
 
 	br_configuration_update(br);