datapath-windows: Fixes in packet created for userspace

A couple of miscellaneous fixes in code that creates a packet for
userspace as well as when we copy the packet to memory specified by
userspace.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>