HDFS-5198. NameNodeRpcServer must not send back DNA_FINALIZE in reply to a cache report.

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HDFS-5195. Prevent passing null pointer to mlock and munlock. Contributed by Chris Nauroth.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-4949@1523093 13f79535-47bb-0310-9956-ffa450edef68