Split ReservedSpace constructor to avoid default parameter

We have this constructor in ReservedSpace:
  // Initialize the reserved space with the given size. If preferred_page_size
  // is set, use this as minimum page size/alignment. This may waste some space
  // if the given size is not aligned to that value, as the reservation will be
  // aligned up to the final alignment in this case.
ReservedSpace(size_t size, size_t preferred_page_size = 0);

Instead of having the default parameter using two constructors would make the implementation more straight forward.

This cleanup is another step towards JDK-8261527.