bridge: fix initial packet flood if !STP

If bridge is configured with no STP and forwarding delay of 0 (which
is typical for virtualization) then when link starts it will flood all
packets for the first 20 seconds.

This bug was introduced by a combination of earlier changes:
  * forwarding database uses hold time of zero to indicate
    user wants to always flood packets
  * optimzation of the case of forwarding delay of 0 avoids the initial
    timer tick

The fix is to just skip all the topology change detection code if
kernel STP is not being used.
Signed-off-by: NStephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_stp.c

@@ -297,6 +297,9 @@ void br_topology_change_detection(struct net_bridge *br)
 {
 	int isroot = br_is_root_bridge(br);
 
+	if (br->stp_enabled != BR_KERNEL_STP)
+		return;
+
 	pr_info("%s: topology change detected, %s\n", br->dev->name,
 		isroot ? "propagating" : "sending tcn bpdu");