NVMe: Export namespace attributes to sysfs

Exposes the NGUID, EUI-64, and NSID to sysfs entries under the disk's
kobject.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Jens Axboe <axboe@fb.com>