bfq: Annotate fall-through in a switch statement

This patch avoids that gcc 7 issues a warning about fall-through
when building with W=1.

Acked-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -3780,6 +3780,7 @@ bfq_set_next_ioprio_data(struct bfq_queue *bfqq, struct bfq_io_cq *bic)
 	default:
 		dev_err(bfqq->bfqd->queue->backing_dev_info->dev,
 			"bfq: bad prio class %d\n", ioprio_class);
+		/* fall through */
 	case IOPRIO_CLASS_NONE:
 		/*
 		 * No prio set, inherit CPU scheduling settings.