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

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

commit c55688698c8cea88dbae42f506eefb5d6150010c upstream.

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

Fixes: e6d3c7b58f9d ("ixgbe: Prevent unsupported configurations with XDP")
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>