Mark pages allocated from the per-CPU cache.

Only free pages to the cache when they were allocated from that cache.
This mitigates rapid fragmentation of physical memory seen during
poudriere's dependency calculation phase. In particular, pages
belonging to broken reservations are no longer freed to the per-CPU
cache, so they get a chance to coalesce with freed pages during the
break. Otherwise, the optimized CoW handler may create object
chains in which multiple objects contain pages from the same
reservation, and the order in which we do object termination means
that the reservation is broken before all of those pages are freed,
so some of them end up in the per-CPU cache and thus permanently
fragment physical memory.

The flag may also be useful for eliding calls to vm_reserv_free_page(),
thus avoiding memory accesses for data that is likely not present
in the CPU caches.

Reviewed by: alc
Discussed with: jeff
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20763