i40e: add double of VLAN header when computing the max MTU

Include the second VLAN HLEN into account when computing the maximum
MTU size as other drivers do.

Fixes: 0c8493d9 ("i40e: add XDP support for pass and drop actions")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Tested-by: Chandan Kumar Rout <chandanx.rout@intel.com> (A Contingent Worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/i40e/i40e_main.c

@@ -2921,7 +2921,7 @@ static int i40e_change_mtu(struct net_device *netdev, int new_mtu)
 	struct i40e_pf *pf = vsi->back;
 
 	if (i40e_enabled_xdp_vsi(vsi)) {
-		int frame_size = new_mtu + ETH_HLEN + ETH_FCS_LEN + VLAN_HLEN;
+		int frame_size = new_mtu + I40E_PACKET_HDR_PAD;
 
 		if (frame_size > i40e_max_xdp_frame_size(vsi))
 			return -EINVAL;