8139too: The maximum MTU should allow for VLAN headers

As pointed out by Ben Hutchings drivers that allow using VLAN have to
provide enough headroom for the VLAN tags.
Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/8139too.c

@@ -112,6 +112,7 @@
 #include <linux/io.h>
 #include <linux/uaccess.h>
 #include <linux/gfp.h>
+#include <linux/if_vlan.h>
 #include <asm/irq.h>
 
 #define RTL8139_DRIVER_NAME   DRV_NAME " Fast Ethernet driver " DRV_VERSION
@@ -182,13 +183,13 @@ static int debug = -1;
 /* Number of Tx descriptor registers. */
 #define NUM_TX_DESC	4
 
-/* max supported ethernet frame size -- must be at least (dev->mtu+14+4).*/
+/* max supported ethernet frame size -- must be at least (dev->mtu+18+4).*/
 #define MAX_ETH_FRAME_SIZE	1792
 
 /* max supported payload size */
-#define MAX_ETH_DATA_SIZE	(MAX_ETH_FRAME_SIZE - ETH_HLEN - ETH_FCS_LEN)
+#define MAX_ETH_DATA_SIZE (MAX_ETH_FRAME_SIZE - VLAN_ETH_HLEN - ETH_FCS_LEN)
 
-/* Size of the Tx bounce buffers -- must be at least (dev->mtu+14+4). */
+/* Size of the Tx bounce buffers -- must be at least (dev->mtu+18+4). */
 #define TX_BUF_SIZE	MAX_ETH_FRAME_SIZE
 #define TX_BUF_TOT_LEN	(TX_BUF_SIZE * NUM_TX_DESC)