nvme: expose subsys attribute to sysfs

We should be exposing the subsystem attributes like 'model' and
'subsysnqn' to sysfs to allow for easier identification of the
subsystem.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>