svn merge -c 1606371 merging from trunk to branch-2 to fix:HDFS-6601. Issues in finalizing rolling upgrade when there is a layout version change.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1606372 13f79535-47bb-0310-9956-ffa450edef68