This obviates the need for several file system implementations to
remember the device on which they are mounted.
Change-Id: Ida8325cf4bcf072e61761cfee34e3f7ed2d750b9
- rename start_vtreefs to run_vtreefs, since the function returns upon
termination these days;
- add get_inode_slots function to retrieve the number of indexed slots;
- add support for extra per-inode data for arbitrary storage.
Change-Id: If2d365d7b478a1cecc9e20fb2b3e70c1a1cf7243
