vm_fault: use iterator for allocation

[dougm]

Summary

Pass an iterator to vm_fault_allocate, so that it can invoke vm_page_alloc_after. In vm_fault_object, use an iterator for a page lookup so that the search can benefit the allocation in vm_fault_allocate. In vm_fault, define an iterator, initialize it only when it might be used, and pass it on to vm_fault_allocate.