HDFS-6638. Merging change r1608905 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1608911 13f79535-47bb-0310-9956-ffa450edef68
move HADOOP-10781 to 2.6

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1608935 13f79535-47bb-0310-9956-ffa450edef68