blk-throttle: Use more suitable time_after check for update of slice_start

There is no need to update tg->slice_start[rw] to start when they are
equal already. So remove "eq" part of check before update slice_start.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Kemeng Shi <shikemeng@huawei.com>
Link: https://lore.kernel.org/r/20221205115709.251489-10-shikemeng@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>