f2fs: fix bad prefetchw of NULL page

For f2fs_read_data_pages, the f2fs_mpage_readpages gets "page == NULL",
so that, the prefetchw(&page->flags) is operated on NULL.

Fixes: f1e88660 ("f2fs: expose f2fs_mpage_readpages")
Signed-off-by: NKinglong Mee <kinglongmee@gmail.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1151,9 +1151,10 @@ static int f2fs_mpage_readpages(struct address_space *mapping,
 
 	for (page_idx = 0; nr_pages; page_idx++, nr_pages--) {
 
-		prefetchw(&page->flags);
 		if (pages) {
 			page = list_last_entry(pages, struct page, lru);
+
+			prefetchw(&page->flags);
 			list_del(&page->lru);
 			if (add_to_page_cache_lru(page, mapping,
 						  page->index,