HDFS-6163. Merge change r1582421 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1582422 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5978. Merge r1582433 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1582434 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10442. Group look-up can cause segmentation fault when certain JNI-based mapping module is used. (Kihwal Lee via jeagles)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1582452 13f79535-47bb-0310-9956-ffa450edef68