sched: Restart fair hrtick after same-task repicks

Fair hrticks are one-shot timers. A same-task repick after hrtick expiry
skips set_next_task_fair() and leaves no hrtick armed for the next fair
preemption point:

    hrtick()
      -> task_tick_fair(..., queued=1)
         -> entity_tick()
            -> resched_curr()
      -> schedule()
         -> pick_task_fair() selects current
         -> put_prev_set_next_task(rq, prev, next)
            -> next == prev
            -> return

Introduce SNT_REPICK for the next == prev path:

    put_prev_set_next_task(rq, prev, next)
      -> next == prev
      -> next->sched_class->set_next_task(rq, next, SNT_REPICK)
         -> set_next_task_fair()
            -> hrtick_start_fair()

set_next_task_fair() skips task-transition work for SNT_REPICK and
restarts the fair hrtick. hrtick_start() records the delay during
schedule(), and hrtick_schedule_exit() rearms the timer.

Do not restart the SCHED_DEADLINE hrtick for SNT_REPICK. Same-task
repicks skip put_prev_task() for both fair and SCHED_DEADLINE, but
pick_task_fair() refreshes the selected fair entity before selecting it
again. hrtick_start_fair() reads that refreshed entity, including with
group scheduling.

The SCHED_DEADLINE picker has no corresponding current-runtime update.
Restarting its hrtick on SNT_REPICK could therefore use stale
dl_se->runtime and delay the next runtime enforcement point.

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Shubhang Kaushik (Ampere) <sh@gentwo.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Link: https://patch.msgid.link/20260917-sched-fair-hrtick-restart-v4-1-4dd1414da81a@gentwo.org

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