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

commit 0967bf83 upstream.

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

Fixes: fabf1bce ("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>