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