vm_page: Add corresponding sysctl knob for loader tunable

The loader tunable 'vm.pgcache_zone_max_pcpu' does not have corresponding
sysctl MIB entry. Add it so that it can be retrieved, and sysctl -T
will also report it correctly.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42138

(cherry picked from commit a55fbda874db31b804490567c69502c891b6ff61)