datapath-windows: Improved offloading on STT tunnel

datapath-windows: Improved offloading on STT tunnel

*Added OvsExtractLayers - populates only the layers field without unnecessary
memory operations for flow part
*If in STT header the flags are 0 then force packets checksums calculation
on receive.
*Ensure correct pseudo checksum is set for LSO both on send and receive.
Linux includes the segment length to TCP pseudo-checksum conforming to
RFC 793 but in case of LSO Windows expects this to be only on
Source IP Address, Destination IP Address, and Protocol.
*Fragment expiration on rx side of STT was set to 30 seconds, but the correct
timeout would be TTL of the packet

Signed-off-by: Paul-Daniel Boca <pboca@cloudbasesolutions.com>
Acked-by: Sairam Venugopal <vsairam@vmware.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>