svn merge -c 1545322 merging from trunk to branch-2 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-2@1545324 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5533. Merge change r1545357 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1545358 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10126. Merge change 1545376 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1545379 13f79535-47bb-0310-9956-ffa450edef68