delayacct: add timestamp of delay max

delayacct: add timestamp of delay max

Problem
=======
Commit 658eb5ab916d ("delayacct: add delay max to record delay peak")
introduced the delay max for getdelays, which records abnormal latency
peaks and helps us understand the magnitude of such delays.  However, the
peak latency value alone is insufficient for effective root cause
analysis.  Without the precise timestamp of when the peak occurred, we
still lack the critical context needed to correlate it with other system
events.

Solution
========
To address this, we need to additionally record a precise timestamp when
the maximum latency occurs.  By correlating this timestamp with system
logs and monitoring metrics, we can identify processes with abnormal
resource usage at the same moment, which can help us to pinpoint root
causes.

Use Case
========
bash-4.4# ./getdelays -d -t 227
print delayacct stats ON
TGID    227
CPU         count     real total  virtual total    delay total  delay average      delay max      delay min      delay max timestamp
               46      188000000      192348334        4098012          0.089ms     0.429260ms     0.051205ms    2026-01-15T15:06:58
IO          count    delay total  delay average      delay max      delay min      delay max timestamp
                0              0          0.000ms     0.000000ms     0.000000ms                    N/A
SWAP        count    delay total  delay average      delay max      delay min      delay max timestamp
                0              0          0.000ms     0.000000ms     0.000000ms                    N/A
RECLAIM     count    delay total  delay average      delay max      delay min      delay max timestamp
                0              0          0.000ms     0.000000ms     0.000000ms                    N/A
THRAS HING   count    delay total  delay average      delay max      delay min      delay max timestamp
                0              0          0.000ms     0.000000ms     0.000000ms                    N/A
COMPACT     count    delay total  delay average      delay max      delay min      delay max timestamp
                0              0          0.000ms     0.000000ms     0.000000ms                    N/A
WPCOPY      count    delay total  delay average      delay max      delay min      delay max timestamp
              182       19413338          0.107ms     0.547353ms     0.022462ms    2026-01-15T15:05:24
IRQ         count    delay total  delay average      delay max      delay min      delay max timestamp
                0              0          0.000ms     0.000000ms     0.000000ms                    N/A

Link: https://lkml.kernel.org/r/20260119100241520gWubW8-5QfhSf9gjqcc_E@zte.com.cn
Signed-off-by: Wang Yaxin <wang.yaxin@zte.com.cn>
Cc: Fan Yu <fan.yu9@zte.com.cn>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: xu xin <xu.xin16@zte.com.cn>
Cc: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>