PHYSPAGE_REFCNT
KASSERT(PHYSPAGE_REFCNT(pg) == 0);
KASSERT(PHYSPAGE_REFCNT(pg) == 0);
KASSERT(PHYSPAGE_REFCNT(pg) < UINT32_MAX);
KASSERT(PHYSPAGE_REFCNT(pg) != 0);
KASSERT(PHYSPAGE_REFCNT(pg) == 0);