Windows: buf_CleanLocked validate cm_scache_t

Windows: buf_CleanLocked validate cm_scache_t

If the cm_scache_t object is not passed in by the caller ensure
that the cm_scache_t has a valid callback.  If the cm_scache_t
has the CM_SCACHEFLAG_DELETED flag set, clear the dirty flag on
the cm_buf_t and do not bother contacting the file server.

Change-Id: If85be550b59765f64aadea4e1882af9430cebcb4
Reviewed-on: http://gerrit.openafs.org/8473
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>