Avoid reloading bucket pointers in uma_vm_zone_stats().

The correctness of per-CPU cache accounting in that function is
dependent on reading per-CPU pointers exactly once. Ensure that
the compiler does not emit multiple loads of those pointers.

Reported and tested by: pho
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22081