ubd: remove ubd_disk_register

Fold it into the only caller to remove lots of references to the
global ubd_devs array.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Richard Weinberger <richard@nod.at>
Link: https://lore.kernel.org/r/20240222072417.3773131-3-hch@lst.de


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