vm_page: add vm_page_clearref() helper

It is supposed to be used for ref_count manipulations when the pages
are owned by an object, but ref_count is used for something else than
the wiring, e.g. PTE population count on the page table page.

Reviewed by: markj
Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D45910