HDFS-4395. In INodeDirectorySnapshottable's constructor, the passed-in dir could be an INodeDirectoryWithSnapshot.  Contributed by Jing Zhao


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1432838 13f79535-47bb-0310-9956-ffa450edef68