vm_glue: use vm_page_alloc_domain_after

Drop the function vm_page_alloc_domain, used only in
vm_thread_stack_back, and replace it with vm_page_alloc_domain_after
there, with the extra mpred argument either computed on the first
iteration or retrieved from previous iterations. Define a function
vm_page_mpred() for computing that first mpred argument.

Reviewed by: bnovkov
Differential Revision: https://reviews.freebsd.org/D47054