uma: fix zone domain overlaying pcpu cache with disabled cpus

UMA zone structures have two arrays at the end which are sized according
to the machine: an array of CPU count length, and an array of NUMA
domain count length. The CPU counting was wrong in the case where some
CPUs are disabled (when mp_ncpus != mp_maxid + 1), and this caused the
second array to be overlaid with the first.

Reported by: olivier
Reviewed by: jeff, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23318