svn merge -c 1534426 from trunk for HDFS-4885. Improve the verifyBlockPlacement() API in BlockPlacementPolicy.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1534427 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd