Fix VPO_UNMANAGED handling in vm_page_reclaim_run() after r353540.

When allocating a replacement page we must clear VPO_UNMANAGED since we
only ever reclaim pages from managed objects. vm_page_replace() does
not handle this for us.

Sprinkle some assertions to help catch this sort of issue.

Reported by: pho
Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22868