vm_fault: use iterator for allocation

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.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D49741