nvme-loop: store the generic nvme_ctrl in set->driver_data

Point the private data to the generic controller structure in preparation
of using the common tagset init/exit code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>