block: fix that iostat can show huge wait time

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I57S8D


CVE: NA

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

There might be a problem that iostat can read less 'nsecs' than the
last time.

1) io is started after 'hd->stat_time' is set.

2) following concurrent scenario:

t1                              t2
                                blk_mq_end_request
				 time1 -> before hd->stat_time

				 blk_account_io_done
part_get_stat_info
 part_set_stat_time
  hd->stat_time = time2
  -> time1 < time2
 blk_mq_in_flight_with_stat
  blk_mq_check_inflight_with_stat
   cmpxchg64()
   -> set stat_time_ns to time2
				  cmpxchg64()
				  -> set stat_time to time1
				  duation = time1 - time2;
				  -> time1 < time2
				  part_stat_add(xx, nsecs, duation)
				  -> problematic

3) Similar concurrent scenario the other way around.

Fix the problem by don't add 'duation' if the calculation might underflow.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>