HDFS-4666. Define ".snapshot" as a reserved inode name so that users cannot create a file/directory with ".snapshot" as the name.  If ".snapshot" is used in a previous version of HDFS, it must be renamed before upgrade; otherwise, upgrade will fail.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1468238 13f79535-47bb-0310-9956-ffa450edef68