block: mq flush: clear flush_rq's tag in flush_end_io()

blk_mq_tag_to_rq() needs to be able to tell if it should return
the original request, or the flush request if we are doing a flush
sequence. Clear the flush tag when IO completes for a flush, since
that is what we are comparing against.
Signed-off-by: NMing Lei <tom.leiming@gmail.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-flush.c

@@ -225,7 +225,7 @@ static void flush_end_io(struct request *flush_rq, int error)
 
 	if (q->mq_ops) {
 		spin_lock_irqsave(&q->mq_flush_lock, flags);
-		q->flush_rq->cmd_flags = 0;
+		q->flush_rq->tag = -1;
 	}
 
 	running = &q->flush_queue[q->flush_running_idx];