e1000e: increase minimum frame size allowed

Setting an MTU value below 68 was disabling the network connection and
would not reconnect until the driver was reloaded.  Prevent changing the
MTU to anything below 68.
Signed-off-by: NBruce Allan <bruce.w.allan@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>

drivers/net/e1000e/netdev.c

@@ -3724,7 +3724,7 @@ static int e1000_change_mtu(struct net_device *netdev, int new_mtu)
 	struct e1000_adapter *adapter = netdev_priv(netdev);
 	int max_frame = new_mtu + ETH_HLEN + ETH_FCS_LEN;
 
-	if ((max_frame < ETH_ZLEN + ETH_FCS_LEN) ||
+	if ((new_mtu < ETH_ZLEN + ETH_FCS_LEN + VLAN_HLEN) ||
 	    (max_frame > MAX_JUMBO_FRAME_SIZE)) {
 		e_err("Invalid MTU setting\n");
 		return -EINVAL;