block: pass a gendisk to blk_queue_update_readahead

.. and rename the function to disk_update_readahead.  This is in
preparation for moving the BDI from the request_queue to the gendisk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20210809141744.1203023-3-hch@lst.de


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