kernel - Fix deep recursion in vm_object_collapse()

kernel - Fix deep recursion in vm_object_collapse()

* vm_object_collapse() will loop but its backing_object sometimes needs
  to be deallocated as well and this can trigger another collapse against
  a different parent object.

* Introduce vm_object_dealloc_list and friends to collect a list of objects
  requiring deallocation so the caller can run the list in a way that avoids
  a deep recursion.

Reported-by: juanfra