mm: memblock: remove return value of memblock_free_all()

No one checks the return value of memblock_free_all().
Make the return value void.

memblock_free_all() is used on mem_init() for each
architecture, and the total count of freed pages will be added
to _totalram_pages variable by calling totalram_pages_add().

so do not need to return total count of freed pages.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>