An rbd clone image that has an overlap with its parent of 0 is
effectively not a layered image at all.  Detect this case and treat
such an image as non-layered.  Issue a warning to be sure the user
knows what's going on.

This resolves:
    http://tracker.ceph.com/issues/5028Signed-off-by: NAlex Elder <elder@inktank.com>
Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>