kern: osd: stop downsizing arrays when the last slot deregisters

[kevans]

Summary

It was noted in D41404 that these reallocations aren't actually
guaranteed to succeed, despite assertions to the contrary. We're
talking relatively small allocations, so just free up the individual
slot to be reused later as needed.

Note that this doesn't track the last active slot as of this moment, but
this could be done later if we find it's worth the complexity for what
little that would allow to be optimized (osd_call, slightly).

While we're here, fix the debug message that indicates which slot we
just allocated when we find an unused one; the slot # is actually one
higher than the index.

[kevans]

sys/kern/kern_osd.c

178–179

Hmm. Should we also NULL out its entries in osd_methods so that we get a guaranteed fault on bogus use?

[markj]

sys/kern/kern_osd.c

178–179

I can't see any downsides to doing that.

[kevans]

sys/kern/kern_osd.c

178–179

I think I'll punt on that until after the branch... what's here is relatively safe, and while NULLing out osd_methods *should be* I think I'll play it conservative and avoid adding code at all this close to branch time.