nvmet: implement id ns for nvm command set

We don't report anything here, but it's a mandatory identification for
nvme 2.1.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Matias Bjørling <matias.bjorling@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>