Pass down requested page size to reserve_memory_special

Once ReservedSpace has been updated to use page sizes rather then a boolean to signal if large pages are used (JDK-8261527) we can use this page size and pass it down to reserve_memory_special to make it clear what page size should be used for the mapping. If only one large page size can be used, this is not really needed. But once JDK-8256155 is integrated passing down the page size will simplify things a lot.