block: pass a gendisk to ->open

->open is only called on the whole device.  Make that explicit by
passing a gendisk instead of the block_device.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: Jack Wang <jinpu.wang@ionos.com>		[rnbd]
Link: https://lore.kernel.org/r/20230608110258.189493-9-hch@lst.de


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