Further constrain the use of per-CPU caches for free pages.

In low memory conditions a significant number of pages may end up stuck
in the caches, and currently these caches cannot be reaped, leading to
spurious memory allocation failures and OOM kills. So:

• Take into account the fact that we may cache up to two full buckets of pages per CPU, not just one.
• Increase the amount of RAM required per CPU to enable the caches.

This is a temporary measure until the page cache management policy is
improved.

PR: 241048
Reported and tested by: Kevin Oberman <rkoberman@gmail.com>
Reviewed by: alc, kib
Discussed with: jeff
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22040