Windows: Return bytes_read count from cm_GetData

The existing interface for cm_GetData takes an input buffer and
a buffer size and no method by which the number of bytes read into
the buffer can be returned.  Add an output parameter to permit it.

Change-Id: I777d2fba850fb39d4323b006e3c79afa5ff4c2ff
Reviewed-on: http://gerrit.openafs.org/9930
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>