blk-throttle: fix lower control under super low iops limit

blk-throttle: fix lower control under super low iops limit

User will configure allowed iops limit in 1s, and calculate_io_allowed()
will calculate allowed iops in the slice by:

limit * HZ / throtl_slice

However, if limit is quite low, the result can be 0, then
allowed IO in the slice is 0, this will cause missing dispatch and
control will be lower than limit.

For example, set iops_limit to 5 with HD disk, and test will found that
iops will be 3.

This is usually not a big deal, because user will unlikely to configure
such low iops limit, however, this is still a problem in the extreme
scene.

Fix the problem by making sure the wait time calculated by
tg_within_iops_limit() should allow at least one IO to be dispatched.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20240618062108.3680835-1-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>