Windows: cm_SetupStoreBIOD compute correct scanEnd

The algorithm used to ensure "chunk size" operations attempts to
enforce aligned chunks.  There are two problems:

 1. an aligned chunk range may extend beyond the end of the file.

 2. an aligned chunk might end before the requested length of the
    active write.

Protect against scanning beyond the end of the file and do not truncate
writes.

Reviewed-on: http://gerrit.openafs.org/10625
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit fb6bc16b335d0e50b49ed85570dbe9fc9adfaee0)

Change-Id: I3c34f2bcbca6978d8205f7cd5bdd240ff6ab56be
Reviewed-on: http://gerrit.openafs.org/10627
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>