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This series has also been squeezed into D47633 to provide an overall view.

Commit message:
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'.