blk-throttle: fix wrong comparation while 'carryover_ios/bytes' is negative

blk-throttle: fix wrong comparation while 'carryover_ios/bytes' is negative

carryover_ios/bytes[] can be negative in the case that ios are
dispatched in the slice in advance, and then configuration is updated.
For example:

1) set iops limit to 1000, and slice start is 0, slice end is 100ms;
2) current time is 0, and 100 ios are dispatched, those ios will not be
   throttled, hence io_disp is 100;
3) still at current time 0, update iops limit to 100, then carryover_ios
   is (0 - 100) = -100;
4) then, dispatch a new io at time 0, the expected result is that this
   io will wait for 1s. The calculation in tg_within_iops_limit:

   io_disp = 0;
   io_allowed = calculate_io_allowed + carryover_ios
	      = 10 + (-100) = -90;
   io won't be throttled if (io_disp + 1 < io_allowed) passed.

Before this patch, in step 4) (io_disp + 1 < io_allowed) is passed,
because -90 for unsigned value is very huge, and such io won't be
throttled.

Fix this problem by checking if 'io/bytes_allowed' is negative first.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20230816012708.1193747-3-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>