HADOOP-10557. Merging change r1603222 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1603226 13f79535-47bb-0310-9956-ffa450edef68
svn merge -c 1603239 merging from trunk to branch-2 to fix:HDFS-6545. Finalizing rolling upgrade can make NN unavailable for a long duration.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1603241 13f79535-47bb-0310-9956-ffa450edef68