blk-throttle: fix io hung due to configuration updates

hulk inclusion
category: bugfix
bugzilla: 186731, https://gitee.com/src-openeuler/kernel/issues/I5HWTF


CVE: NA

--------------------------------

If new configuration is submitted while a bio is throttled, then new
waiting time is recaculated regardless that the bio might aready wait
for some time:

tg_conf_updated
 throtl_start_new_slice
  tg_update_disptime
  throtl_schedule_next_dispatch

Then io hung can be triggered by always submmiting new configuration
before the throttled bio is dispatched.

Fix the problem by respecting the time that throttled bio aready waited.
In order to do that, instead of start new slice in tg_conf_updated(),
just update 'bytes_disp' and 'io_disp' based on the new configuration.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>