Cleanup OBJ_ONEMAPPING management.

Cleanup OBJ_ONEMAPPING management.

vm_map.c:
	Don't set OBJ_ONEMAPPING on arbitrary vm objects.  Only default
	and swap type vm objects should have it set.  vm_object_deallocate
	already handles these cases.

vm_object.c:
	If OBJ_ONEMAPPING isn't already clear in vm_object_shadow,
	we are in trouble.  Instead of clearing it, make it
	an assertion that it is already clear.