Windows: clear mountPointStringp on status change

Windows: clear mountPointStringp on status change

If the data version of the cm_scache_t changes, we must clear
the mountPointString.  Otherwise, if the object is a mountpoint
or symlink, cm_HandleLink() will not attempt to build the new
target string.   The Windows CM never updates the contents of
a mountpoint or symlink.  It always destroys the old and creates
a new one.  However, the Unix CM simply updates the link data.
Apparently, the Windows clients have never noticed.

Change-Id: If2297ed0a323411df13b7828d11dbb653669d351
Reviewed-on: http://gerrit.openafs.org/3552
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>