HADOOP-10087. UserGroupInformation.getGroupNames() fails to return primary group first when JniBasedUnixGroupsMappingWithFallback is used (cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1550230 13f79535-47bb-0310-9956-ffa450edef68