bcachefs: Fix for __readahead_batch getting partial batch

We were incorrectly ignoring the return value of __readahead_batch,
leading to a null ptr deref in __bch2_page_state_create().
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io.c

@@ -667,7 +667,7 @@ static int readpages_iter_init(struct readpages_iter *iter,
 	if (!iter->pages)
 		return -ENOMEM;
 
-	__readahead_batch(ractl, iter->pages, nr_pages);
+	nr_pages = __readahead_batch(ractl, iter->pages, nr_pages);
 	for (i = 0; i < nr_pages; i++) {
 		__bch2_page_state_create(iter->pages[i], __GFP_NOFAIL);
 		put_page(iter->pages[i]);