Windows: AFSCleanup extent processing

Windows: AFSCleanup extent processing

1. Perform a CcFlushCache() any time the file is cached
   and the Context Control Block indicates that the handle
   has FILE_WRITE_DATA permission.

2. Perform an AFSFlushExtents() whenever there are dirty
   extents and the handle has FILE_WRITE_DATA permission.
   No point flushing the extents if the AuthGroup does not
   have write permission.  Another Ccb must exist that does
   have write permission.

Change-Id: I3ece011b484c12e7dc936b81c272ba6a42f6c7d6
Reviewed-on: http://gerrit.openafs.org/6399
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>