net: dsa: don't call switchdev_bridge_port_unoffload for unoffloaded bridge ports

For ports that have a NULL dp->bridge_dev, dsa_port_to_bridge_port()
also returns NULL as expected.

Issue #1 is that we are performing a NULL pointer dereference on brport_dev.

Issue #2 is that these are ports on which switchdev_bridge_port_offload
has not been called, so we should not call switchdev_bridge_port_unoffload
on them either.

Both issues are addressed by checking against a NULL brport_dev in
dsa_port_pre_bridge_leave and exiting early.

Fixes: 2f5dc00f ("net: bridge: switchdev: let drivers inform which bridge ports are offloaded")
Signed-off-by: NVladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dsa/port.c

@@ -373,6 +373,10 @@ void dsa_port_pre_bridge_leave(struct dsa_port *dp, struct net_device *br)
 {
 	struct net_device *brport_dev = dsa_port_to_bridge_port(dp);
 
+	/* Don't try to unoffload something that is not offloaded */
+	if (!brport_dev)
+		return;
+
 	switchdev_bridge_port_unoffload(brport_dev, dp,
 					&dsa_slave_switchdev_notifier,
 					&dsa_slave_switchdev_blocking_notifier);