ubd: remove the ubd_gendisk array

And add a disk pointer to the ubd structure instead to keep all
the per-device information together.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Richard Weinberger <richard@nod.at>
Link: https://lore.kernel.org/r/20240222072417.3773131-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>