blk-mq: fix a hung issue when fsync

Florian reported a io hung issue when fsync(). It should be
triggered by following race condition.

data + post flush         a flush

blk_flush_complete_seq
  case REQ_FSEQ_DATA
    blk_flush_queue_rq
    issued to driver      blk_mq_dispatch_rq_list
                            try to issue a flush req
                            failed due to NON-NCQ command
                            .queue_rq return BLK_STS_DEV_RESOURCE

request completion
  req->end_io // doesn't check RESTART
  mq_flush_data_end_io
    case REQ_FSEQ_POSTFLUSH
      blk_kick_flush
        do nothing because previous flush
        has not been completed
     blk_mq_run_hw_queue
                              insert rq to hctx->dispatch
                              due to RESTART is still set, do nothing

To fix this, replace the blk_mq_run_hw_queue in mq_flush_data_end_io
with blk_mq_sched_restart to check and clear the RESTART flag.

Fixes: bd166ef (blk-mq-sched: add framework for MQ capable IO schedulers)
Reported-by: Florian Stecker <m19@florianstecker.de>
Tested-by: Florian Stecker <m19@florianstecker.de>
Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -335,7 +335,7 @@ static void mq_flush_data_end_io(struct request *rq, blk_status_t error)
 	blk_flush_complete_seq(rq, fq, REQ_FSEQ_DATA, error);
 	spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
 
-	blk_mq_run_hw_queue(hctx, true);
+	blk_mq_sched_restart(hctx);
 }
 
 /**