fix same available space policy in AvailableSpaceVolumeChoosingPolicy

if all the volumes have the same available space. for example 1 MB and dfs.datanode.available-space-volume-choosing-policy.balanced-space-threshold is set 0, which means we should treat all the volumes equally when we choose available volumes.but currently not, we can fix it in AvailableSpaceVolumeChoosingPolicy.