svn merge -c 1545322 merging from trunk to branch-0.23 to fix HDFS-5266. Datanode cannot roll back to previous layout version.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.23@1545326 13f79535-47bb-0310-9956-ffa450edef68