nvmet: allow overriding the NVMe VS via configfs

Allow overriding the announced NVMe Version of a via configfs.

This is particularly helpful when debugging new features for the host
or target side without bumping the hard coded version (as the target
might not be fully compliant to the announced version yet).

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Guan Junxiong <guanjunxiong@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>