block: simplify blktrace sysfs attribute creation

Add the trace attributes to the default gendisk attributes, just like
we already do for partitions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220628171850.1313069-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>