Chunk server: do not attempt to queue disk io request when running stale chunks queue, if the chunk directory disk io queue has been stopped already, as the corresponding directory prefix is already removed from the disk io queues. Under normal circumstances an attempt to queue an io request only consumes cpu. With testing, when any disk io request failure is treated as fatal error, such extraneous requests cause test failures. For example such problem appears with evacuation testing with endurance test.