s390/mm: get estimated free pages by memblock api

Instead of getting estimated free pages from memblock directly, we have
introduced an API, memblock_estimated_nr_free_pages(), which is more
friendly for users.

Just replace it with new API, no functional change.
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Mike Rapoport <rppt@kernel.org>
CC: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240808001415.6298-3-richard.weiyang@gmail.comSigned-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

arch/s390/mm/init.c

@@ -62,7 +62,7 @@ EXPORT_SYMBOL(zero_page_mask);
 
 static void __init setup_zero_pages(void)
 {
-	unsigned long total_pages = PHYS_PFN(memblock_phys_mem_size() - memblock_reserved_size());
+	unsigned long total_pages = memblock_estimated_nr_free_pages();
 	unsigned int order;
 	struct page *page;
 	int i;