block: fold register_disk into device_add_disk

There is no real reason these should be separate.  Also simplify the
groups assignment a bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210818144542.19305-3-hch@lst.de


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