block: pass a gendisk to bdev_del_partition

bdev_del_partition can only operate on the whole device.  Make that clear
by passing a gendisk instead of a block_device.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210810154512.1809898-4-hch@lst.de


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