Prefer the use of vm_page_domain() to vm_phys_domain()

[alc]

Summary

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. Since both implementations are inlined, this change to just two call sites reduces the code size on amd64 by 320 bytes.

[kib]

As a note, this code goes with pv_locks patch (AFAIR).