igb: Enable TSO for stacked vlan

As datasheets for igb (I210, I350, 82576, etc.) say, maclen can be from
14 to 127, which is enough for reasonable number of vlan tags.
My netperf test showed I350's TSO works pretty fine with multiple vlans.

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>