f2fs: use ->writepage in sync_meta_pages

This patch uses ->writepage of meta mapping in sync_meta_pages instead of
f2fs_write_meta_page, by this way, in its caller we can ignore any changes
(e.g. changing name) of this registered function.
Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/checkpoint.c

@@ -276,7 +276,7 @@ long sync_meta_pages(struct f2fs_sb_info *sbi, enum page_type type,
 			if (!clear_page_dirty_for_io(page))
 				goto continue_unlock;
 
-			if (f2fs_write_meta_page(page, &wbc)) {
+			if (mapping->a_ops->writepage(page, &wbc)) {
 				unlock_page(page);
 				break;
 			}