When reporting stats for the backing chain some of them make sense only
for the topmost entry as they are actually tied to the frontend device.
We unfortunately can't change that fact, but we can stop reporting all
zero stats for the backing chain members where they don't make any
sense.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>