Batch updates to a reservation's population map.

Each such update causes the reservation to be requeued in the per-domain
partially populated reservation queue, preserving LRU. The queue is
protected by the per-domain reservation lock. Now that
vm_reserv_extend() and vm_reserv_alloc_page() opportunistically return
multiple consecutive pages to support vm_page_alloc_pages_after(), it is
beneficial to requeue the reservation only after all popmap updates are
finished, rather than requeuing once per page. This also helps with
contiguous allocation requests, which may also return multiple pages
depending on the request constraints.