uma: Avoid excessive per-CPU draining

After commit 389a3fa693ef, uma_reclaim_domain(UMA_RECLAIM_DRAIN_CPU)
calls uma_zone_reclaim_domain(UMA_RECLAIM_DRAIN_CPU) twice on each zone
in addition to globally draining per-CPU caches. This was unintended
and is unnecessarily slow; in particular, draining per-CPU caches
requires binding to each CPU.

Stop draining per-CPU caches when visiting each zone, just do it once in
pcpu_cache_drain_safe() to minimize the amount of expensive sched_bind()
calls.

Fixes: 389a3fa693ef ("uma: Add UMA_ZONE_UNMANAGED")
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: gallatin, kib
Differential Revision: https://reviews.freebsd.org/D49349

(cherry picked from commit f506d5af50fccc37f5aa9fe090e9a0d5f05506c8)