datapath-windows: Compute checksums for VXLAN inner packets.

datapath-windows: Compute checksums for VXLAN inner packets.

Windows does not support VXLAN hardware offloading.

Currently we do not compute IP/TCP/UDP checksums for the inner packet. This
patch computes the checksums mentioned above in regards with the enabled
settings.

i.e. if IP checksum offloading is enabled for the inner packet we compute it.
The same applies for TCP and UDP packets.

This patch also revizes the computation of ones' complement over different
memory blocks, in the case the lengths are odd.

Also per documentation:
https://msdn.microsoft.com/en-us/library/windows/hardware/ff568840%28v=vs.85%29.aspx
set the TCP flags FIN and PSH only for the last segment in the case LSO is
enabled.

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Sairam Venugopal <vsairam@vmware.com>
Acked-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>