uma: Introduce per-domain reclamation functions

uma: Introduce per-domain reclamation functions

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.

Currently the new KPIs are unused, so there should be no functional
change.

Reviewed by:	mav
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D29685