bugfix: it must wait for checkpointing to keep consistency.
disable logs in walb_ioctl().
consolidate logs in prepare_wdev().
improve consistency check.
bugfix: forget to check overflow at device creation.
assign NULL for strong check.
fix: set WALB_STATE_FINALIZE flag before calling del_gendisk().
fix: use is_wdev_dying() to return -ENODEV for bios submitted after starting device finalization.