[PATCH 10/17] nvme: switch controller refcounting to use struct device

[Sagi Grimberg]

> -	if (!kref_get_unless_zero(&ctrl->ctrl.kref))
> -		return -EBUSY;
> +	nvme_get_ctrl(&ctrl->ctrl);

Given that we take this reference before we are protected with
the state change I think this should still be get_unless_zero.

Maybe we can introduce get_device_unless_zero() for this?