blk-mq: mark flush request as IDLE in flush_end_io()

Mark flush request as IDLE in its .end_io(), aligning it with how normal
requests behave. The flush request stays in in-flight tags if we're not
using an IO scheduler, so we need to change its state into IDLE.
Otherwise, we will hang in blk_mq_tagset_wait_completed_request() during
error recovery because flush the request state is kept as COMPLETED.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Cc: Chao Leng <lengchao@huawei.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -225,6 +225,7 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
 	/* release the tag's ownership to the req cloned from */
 	spin_lock_irqsave(&fq->mq_flush_lock, flags);
 
+	WRITE_ONCE(flush_rq->state, MQ_RQ_IDLE);
 	if (!refcount_dec_and_test(&flush_rq->ref)) {
 		fq->rq_status = error;
 		spin_unlock_irqrestore(&fq->mq_flush_lock, flags);