Revision 353734

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