writeback: don't use schedule_timeout() without setting runstate

Just use schedule_timeout_interruptible(), saves a call to
set_current_state().
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

mm/page-writeback.c

@@ -561,7 +561,7 @@ static void balance_dirty_pages(struct address_space *mapping)
 		if (pages_written >= write_chunk)
 			break;		/* We've done our duty */
 
-		schedule_timeout(1);
+		schedule_timeout_interruptible(1);
 	}
 
 	if (bdi_nr_reclaimable + bdi_nr_writeback < bdi_thresh &&