pktcdvd: use class_groups instead of class_attrs

The class_attrs pointer is long depreciated, and is about to be finally
removed, so move to use the class_groups pointer instead.

Cc: <linux-block@vger.kernel.org>
Acked-by: Jens Axboe <axboe@fb.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Bart Van Assche <Bart.VanAssche@sandisk.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>