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.

Change-Id: Ibe6caebd78f73d2c93bfef0dcebef379ca843994
Reviewed-on: http://gerrit.openafs.org/10625
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>