- The leaked blocks are defined as allocated but not used ones,
i.e., expected ref-count == 0 and actual ref-count <= 2.
- Blocks with expected ref-count > 0, or actual ref-count > 2,
are treated as unrepairable leaks.
- The fixing processing won't be executed if there's any
unrepairable leak or errors.
- The fixing process implicitly clearing the superblock
needs_check flag if there's no error.
49efa06ffa