intrng: switch from MAXCPU to mp_ncpus

MAXCPU could be on the large side, while the hardware may not have
many processors at all. As such only allocate counters for processors
which actually exist, rather than always allocating for the maximum
potentially allowed number.

Reviewed by: markj, emaste
Differential Revision: https://reviews.freebsd.org/D41462

(cherry picked from commit 2f0b059eeafc545a4ab1835d6a5290e1e2ebb47f)