datapath-windows: Support for IPv6 in TCP segmentation

datapath-windows: Support for IPv6 in TCP segmentation

When a packet which needs segmentation is received, the header for
each segment is being calculated, i.e. IP length, checksum, TCP seq,
TCP checksum.

The problem with the current code is that it wrongly assumes that
the Ethernet frame payload is always an IPv4 packet.

This patch checks the EtherType field of the Ethernet frame to see
which protocol is encapsulated in its payload, IPv4 or IPv6, and
calculates the segment's header accordingly.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Co-authored-by: Sairam Venugopal <vsairam@vmware.com>
Reported-by: Sairam Venugopal <vsairam@vmware.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/105
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>