svn merge -c 1429653 from trunk for HDFS-4351.  In BlockPlacementPolicyDefault.chooseTarget(..), numOfReplicas needs to be updated when avoiding stale nodes.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1429654 13f79535-47bb-0310-9956-ffa450edef68