bridge: reset bridge mtu after deleting an interface

On adding an interface br_add_if() sets the MTU to the min of
all the interfaces. Do the same thing on removing an interface too
in br_del_if.

Signed-off-by: Venkat Venkatsubra <venkat.x.venkatsubra@oracle.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_if.c

@@ -563,6 +563,8 @@ int br_del_if(struct net_bridge *br, struct net_device *dev)
 	 */
 	del_nbp(p);
 
+	dev_set_mtu(br->dev, br_min_mtu(br));
+
 	spin_lock_bh(&br->lock);
 	changed_addr = br_stp_recalculate_bridge_id(br);
 	spin_unlock_bh(&br->lock);