block: fix leaking minors of hidden disks

The major/minor of a hidden gendisk is not propagated to the block
device because it is never registered using bdev_add.  But the lack of
bd_dev also causes the dynamic major minor number not to be freed.
Assign bd_dev manually to ensure the dynamic major minor gets freed.

Based on a patch by Keith Busch.

Fixes: 8ddcd653 ("block: introduce GENHD_FL_HIDDEN")
Reported-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20221010131857.748129-1-hch@lst.de


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