[RFC v1 1/4] nvmet-trace: avoid dereferencing pointer too early

[Christoph Hellwig]

> +static inline u16 nvmet_req_to_ctrl_id(struct nvmet_req *req)
>  {
> -	return req->sq->ctrl;
> +	if (!req->sq || !req->sq->ctrl)
> +		return 0;
> +	return req->sq->ctrl->cntlid;

Can you add a comment here why we have this check?