HDFS-4500. Refactor snapshot INode methods.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1446355 13f79535-47bb-0310-9956-ffa450edef68
HDFS-4487. Fix snapshot diff report for HDFS-4446.  Contributed by Jing Zhao


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1446385 13f79535-47bb-0310-9956-ffa450edef68