uma: Introduce per-domain reclamation functions

[markj]

Summary

Make it possible to reclaim items from a specific NUMA domain.

- Add uma_zone_reclaim_domain() and uma_reclaim_domain().
- Permit parallel reclamations. Use a counter instead of a flag to
synchronize with zone_dtor().
- Use the zone lock to protect cache_shrink() now that parallel reclaims
can happen.
- Add a sysctl that can be used to trigger reclamation from a specific
domain.

There is one issue I am not sure how to resolve. uma_reclaim()
currently tries to put pressure on bucket sizes. If multiple page
daemons start reclaiming their respective UMA caches concurrently, we'd
be putting extra pressure on the bucket size.

[mav]

Looks good to me.

We should definitely reconsider whole bucket size pressure mechanism at some point, but I don't think we should worry about extra pressure right now. I suspect we may have insufficient pressure now. And even if we overpressure it, it should quickly compensate back if there is a lock contention. And if there is no lock contention, then shrink was reasonable. I just think we need some mechanism to create small constant pressure in case there is no memory pressure right now, but it may appear later. We should keep the subsystem fit, not overbloated, even if it is not stressed.