Windows: AFSCleanupFile always flush on last handle

Do not rely on a count of dirty extents to determine if the
service should flush a file during a cleanup operation.  Simply
because there are no dirty extents held by the redirector does
not imply that the service has no dirty buffers for the file.

Change-Id: Ifc7c139a587413626eae145f343c4295dd94f28a
Reviewed-on: http://gerrit.openafs.org/8971
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>