powerpc: mem_init should call memblock_is_reserved with phys_addr_t

This has been broken for a while but hasn't been an issue until
now because nobody was reserving regions at high addresses.
Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/mem.c

@@ -337,8 +337,9 @@ void __init mem_init(void)
 
 		highmem_mapnr = lowmem_end_addr >> PAGE_SHIFT;
 		for (pfn = highmem_mapnr; pfn < max_mapnr; ++pfn) {
+			phys_addr_t paddr = (phys_addr_t)pfn << PAGE_SHIFT;
 			struct page *page = pfn_to_page(pfn);
-			if (memblock_is_reserved(pfn << PAGE_SHIFT))
+			if (memblock_is_reserved(paddr))
 				continue;
 			ClearPageReserved(page);
 			init_page_count(page);