block: Update a comment of disk statistics

>From commit 074a7aca ("block: move stats from disk to part0"),
we know that:

* {disk|all}_stat_*() are gone.

* disk_stat_lock/unlock() are renamed to part_stat_lock/unlock().

Therefore, outdated comments should be updated accordingly.

Fixes: 074a7aca ("block: move stats from disk to part0")
Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>