datapath-windows: Refactor sofware offloads and mss

datapath-windows: Refactor sofware offloads and mss

The purpose of this patch is to refactor the software offloads found in
the VXLAN and GRE code and also to refactor how the maximmum segment
size for a given NBL is obtained.

This patch introduces two functions OvsApplySWChecksumOnNB and OVSGetTcpMSS.

OVSGetTcpMSS - will return the mss found in a given NBL.

OvsApplySWChecksumOnNB - will compute and set software offloads for a given
                         NBL.

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Sorin Vinturis <svinturis at cloudbasesolutions.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>