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.

Reviewed-on: http://gerrit.openafs.org/3552
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit bb4c0234b2fa5ae91097b6c40cbfda6ed7484eb0)

Change-Id: Icabba0b9bb6a1484d3ee3bc21dc6a39343e5491c
Reviewed-on: http://gerrit.openafs.org/3803
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>