Hannes just spotted this:
static int nvme_queue_rq(struct blk_mq_hw_ctx *hctx,
                         const struct blk_mq_queue_data *bd)
{
[...]
        __nvme_submit_cmd(nvmeq, &cmnd);
        nvme_process_cq(nvmeq);
        spin_unlock_irq(&nvmeq->q_lock);
        return BLK_MQ_RQ_QUEUE_OK;
out_cleanup_iod:
        nvme_free_iod(dev, req);
out_free_cmd:
        nvme_cleanup_cmd(req);
        return ret;
}

So we're draining the CQ on submit. This of cause makes polling for
completions in the IRQ handler rather pointless as we already did in the
submission path.

I think you missed:
http://git.infradead.org/nvme.git/commit/49c91e3e09dc3c9dd1718df85112a8cce3ab7007