Prefer the use of vm_page_domain() to vm_phys_domain().

When we already have the vm page in hand, use vm_page_domain() instead
of vm_phys_domain(). The former has a trivial constant-time
implementation whereas the latter iterates over the mem_affinity array.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D28005