HDFS-6478. Merge r1611410 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1611412 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10732. Fix locking in credential update. (Ted Yu via omalley)


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1611416 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10733. Fix potential null dereference in CredShell. (Ted Yu via
omalley)


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1611421 13f79535-47bb-0310-9956-ffa450edef68