block: fix use-after-free in block flush handling

A recent commit reused the original request flags for the flush
queue handling. However, for some of the kick flush cases, the
original request was already completed. This caused a use after
free, if blk-mq wasn't used.

Fixes: 84fca1b0 ("block: pass failfast and driver-specific flags to flush requests")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -169,9 +169,11 @@ static bool blk_flush_complete_seq(struct request *rq,
 	struct request_queue *q = rq->q;
 	struct list_head *pending = &fq->flush_queue[fq->flush_pending_idx];
 	bool queued = false, kicked;
+	unsigned int cmd_flags;
 
 	BUG_ON(rq->flush.seq & seq);
 	rq->flush.seq |= seq;
+	cmd_flags = rq->cmd_flags;
 
 	if (likely(!error))
 		seq = blk_flush_cur_seq(rq);
@@ -212,7 +214,7 @@ static bool blk_flush_complete_seq(struct request *rq,
 		BUG();
 	}
 
-	kicked = blk_kick_flush(q, fq, rq->cmd_flags);
+	kicked = blk_kick_flush(q, fq, cmd_flags);
 	return kicked | queued;
 }