mm: fix integer as NULL pointer warnings

mm/hugetlb.c:207:11: warning: Using plain integer as NULL pointer
Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -204,7 +204,7 @@ static struct page *alloc_fresh_huge_page_node(int nid)
 	if (page) {
 		if (arch_prepare_hugepage(page)) {
 			__free_pages(page, HUGETLB_PAGE_ORDER);
-			return 0;
+			return NULL;
 		}
 		set_compound_page_dtor(page, free_huge_page);
 		spin_lock(&hugetlb_lock);