Merge 1552256 from trunk: HDFS-5540. Fix intermittent failure in TestBlocksWithNotEnoughRacks. (Binglin Chang via junping_du)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1552278 13f79535-47bb-0310-9956-ffa450edef68