[NET]: Allow large MTU on dummy net device.

It is useful for testing to allow larger MTU value to be set on
the dummy network device. The current code limits it to valid Ether mtu's.
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@redhat.com>

drivers/net/dummy.c

@@ -78,6 +78,7 @@ static void __init dummy_setup(struct net_device *dev)
 	/* Fill in device structure with ethernet-generic values. */
 	ether_setup(dev);
 	dev->tx_queue_len = 0;
+	dev->change_mtu = NULL;
 	dev->flags |= IFF_NOARP;
 	dev->flags &= ~IFF_MULTICAST;
 	SET_MODULE_OWNER(dev);