kernel - Rearrange page adds in vm_add_new_page()

kernel - Rearrange page adds in vm_add_new_page()

* We were previously alternating between the head and tail of the PQ_FREE
  queues when adding new vm pages, but not zeroing the pages being placed
  at the tail.

* Instead of alternating place 1/16 of the new pages on the tail of the
  queue, zero them, and properly mark them PG_ZERO.

* This should reduce physical page fragmentation during early boot when
  drivers need to allocate contiguous low phys-memory pages.