Unify compression headers (chained and unchained) into a single struct
so we can use it for the initial compression transform header
interchangeably.

Also make the OriginalPayloadSize field to be always visible in the
compression payload header, and have callers subtract its size when not
needed.

Rename the related structs to match the naming convetion used in the
other SMB2 structs.
Signed-off-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>