mm: make __swap_writepage() use bio_set_op_attrs()

Cleaner than manipulating bio->bi_rw flags directly.
Signed-off-by: Jens Axboe <axboe@fb.com>

mm/page_io.c

@@ -319,9 +319,10 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 		ret = -ENOMEM;
 		goto out;
 	}
-	bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
 	if (wbc->sync_mode == WB_SYNC_ALL)
-		bio->bi_rw |= REQ_SYNC;
+		bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_SYNC);
+	else
+		bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
 	count_vm_event(PSWPOUT);
 	set_page_writeback(page);
 	unlock_page(page);