The diff adds vm_page_alloc_noobj() and vm_page_alloc_noobj_domain().
These mostly correspond to vm_page_alloc() and vm_page_alloc_domain()
when no VM object is specified, with the exception that they handle
VM_ALLOC_ZERO by zeroing the page, rather than by preserving PG_ZERO.
This simplifies callers and makes it harder to accidentally omit a
pmap_zero_page() call. It also permits simplification of the
Since the new interface is similar to vm_page_alloc_freelist(),
implement both of them using a common backend allocator function.
No functional change intended.