blk_iocost: make read-only static array vrate_adj_pct const

The static array vrate_adj_pct is read-only, so make it const as
well.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20240911214124.197403-1-colin.i.king@gmail.com


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