nvmet: declare 2.1 version compliance

The target driver implements all the mandatory logs, identifications,
features, and properties up to nvme sepcification 2.1.

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