CONFIG_BLK_DEV_NULL_BLK_FAULT_INJECTION is enabled in the kernel config,
so null_queue_rq() will return BLK_STS_RESOURCE or BLK_STS_DEV_RESOURCE
for some requests, which have been marked as IN_FLIGHT status.
Then null_queue_rqs() put these requests in the rqlist and return back,
blk-mq will try to queue them individually once again, caused the warning
"WARN_ON_ONCE(blk_mq_rq_state(rq) != MQ_RQ_IDLE)" in blk_mq_start_request().
So handling of return value of null_queue_rq() in null_queue_rqs() is wrong,
maybe we should __blk_mq_requeue_request() for these requests, before
adding them in the rqlist?