kernel - Beef up vm_contig allocation checks

kernel - Beef up vm_contig allocation checks

* Beef up vm_contig allocation checks by also requiring the PQ_CACHE page
  to not be held or wired or unmanaged, and assert that it is not mapped
  or dirty.

* When utilizing PQ_FREE pages ignore any held pages and assert that
  the pages are not dirty in addition to not having an object.

* Do not clear PG_SBUSY when allocating a page (though it shouldn't be set
  anyway, we still shouldn't unconditionally clear the bit).