net: dsa: Restore MTU on master device on unload

A previous change tries to set the MTU on the master device to take
into account the DSA overheads. This patch tries to reset the master
device back to the default MTU.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/master.c

@@ -172,6 +172,18 @@ void dsa_master_set_mtu(struct net_device *dev, struct dsa_port *cpu_dp)
 	rtnl_unlock();
 }
 
+static void dsa_master_reset_mtu(struct net_device *dev)
+{
+	int err;
+
+	rtnl_lock();
+	err = dev_set_mtu(dev, ETH_DATA_LEN);
+	if (err)
+		netdev_dbg(dev,
+			   "Unable to reset MTU to exclude DSA overheads\n");
+	rtnl_unlock();
+}
+
 int dsa_master_setup(struct net_device *dev, struct dsa_port *cpu_dp)
 {
 	dsa_master_set_mtu(dev,  cpu_dp);
@@ -190,6 +202,7 @@ int dsa_master_setup(struct net_device *dev, struct dsa_port *cpu_dp)
 void dsa_master_teardown(struct net_device *dev)
 {
 	dsa_master_ethtool_teardown(dev);
+	dsa_master_reset_mtu(dev);
 
 	dev->dsa_ptr = NULL;