powerpc/mm: Fix a WARN_ON() with CONFIG_DEBUG_PAGEALLOC and CONFIG_DEBUG_VM

Set need to call __set_pte_at() and not set_pte_at() from __change_page_attr()
since the later will perform checks with CONFIG_DEBUG_VM that aren't suitable
to the way we override an existing PTE. (More specifically, it doesn't let
you write over a present PTE).
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/pgtable_32.c

@@ -382,7 +382,7 @@ static int __change_page_attr(struct page *page, pgprot_t prot)
 		return 0;
 	if (!get_pteptr(&init_mm, address, &kpte, &kpmd))
 		return -EINVAL;
-	set_pte_at(&init_mm, address, kpte, mk_pte(page, prot));
+	__set_pte_at(&init_mm, address, kpte, mk_pte(page, prot), 0);
 	wmb();
 #ifdef CONFIG_PPC_STD_MMU
 	flush_hash_pages(0, address, pmd_val(*kpmd), 1);