block: merge block_ioctl into blkdev_ioctl

Simplify the ioctl path and match the code structure on the compat side.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211012104450.659013-4-hch@lst.de


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