It isn't really needed, since common jail code destroys jail OSD storage
at jail destruction (via osd_jail_exit()), triggering our destructor
dealloc_osd(). Leveraging this mechanism is arguably even better as it
causes deallocation to always happen without the 'allprison_lock' lock.
While here, make the static definition of 'methods' top-level, renaming
it to 'osd_methods'.