igc: Add GSO partial support

Partial generic segmentation offload is a hybrid between TSO and GSO.
What is effectively does is take advantage of certain traits of TCP and
tunnels so that instead of having to rewrite the packet headers for each
segment only in the inner-most transport header and possible the outer-most
network header need to be updated.
This allows devices that do not support tunnel offload or tunnels
offloads with checksum to still make use of segmentation.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>