Should log a warning message when users try to make certain directories encryption zone

If a user tries to make the entire /user directory an encryption zone, and if trash is enabled, there will be problem when the user tries to delete unencrypted files outside /user. The problem will happen even with the fix in HDFS-8831. So we should log a WARN message when users try to make such directories encryption zone. Such directories include:
/user,
/user/$user
/user/$user/.Trash

Thanks xyao for the offline discussion.