block: remove disk_update_readahead

Mark blk_apply_bdi_limits non-static and open code disk_update_readahead
in the only caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240626142637.300624-7-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>