Windows: AFSCachedWrite reset LastServerFlush when Forced

Windows: AFSCachedWrite reset LastServerFlush when Forced

If the ForceFlush parameter to AFSCachedWrite is set to TRUE,
the Fcb->Specific.File.LastServerFlush value must be reset to
ensure that the AFSPrimaryVolumeWorkerThread will trigger an
extent flush on its next pass.  The LastServerFlush value will
not be reset by AFSNonCachedWrite() when called as a result of
a CcFlushCache() call as it appears to be PagingIo.

Change-Id: I72c368e05f30f04b13b2c6d098bdb1da578ac82c
Reviewed-on: http://gerrit.openafs.org/8717
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>