sched/dlserver: Fix dlserver double enqueue

dlserver can get dequeued during a dlserver pick_task due to the delayed
deueue feature and this can lead to issues with dlserver logic as it
still thinks that dlserver is on the runqueue. The dlserver throttling
and replenish logic gets confused and can lead to double enqueue of
dlserver.

Double enqueue of dlserver could happend due to couple of reasons:

Case 1
------

Delayed dequeue feature[1] can cause dlserver being stopped during a
pick initiated by dlserver:
  __pick_next_task
   pick_task_dl -> server_pick_task
    pick_task_fair
     pick_next_entity (if (sched_delayed))
      dequeue_entities
       dl_server_stop

server_pick_task goes ahead with update_curr_dl_se without knowing that
dlserver is dequeued and this confuses the logic and may lead to
unintended enqueue while the server is stopped.

Case 2
------
A race condition between a task dequeue on one cpu and same task's enqueue
on this cpu by a remote cpu while the lock is released causing dlserver
double enqueue.

One cpu would be in the schedule() and releasing RQ-lock:

current->state = TASK_INTERRUPTIBLE();
        schedule();
          deactivate_task()
            dl_stop_server();
          pick_next_task()
            pick_next_task_fair()
              sched_balance_newidle()
                rq_unlock(this_rq)

at which point another CPU can take our RQ-lock and do:

        try_to_wake_up()
          ttwu_queue()
            rq_lock()
            ...
            activate_task()
              dl_server_start() --> first enqueue
            wakeup_preempt() := check_preempt_wakeup_fair()
              update_curr()
                update_curr_task()
                  if (current->dl_server)
                    dl_server_update()
                      enqueue_dl_entity() --> second enqueue

This bug was not apparent as the enqueue in dl_server_start doesn't
usually happen because of the defer logic. But as a side effect of the
first case(dequeue during dlserver pick), dl_throttled and dl_yield will
be set and this causes the time accounting of dlserver to messup and
then leading to a enqueue in dl_server_start.

Have an explicit flag representing the status of dlserver to avoid the
confusion. This is set in dl_server_start and reset in dlserver_stop.

Fixes: 63ba8422f876 ("sched/deadline: Introduce deadline servers")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Marcel Ziswiler <marcel.ziswiler@codethink.co.uk> # ROCK 5B
Link: https://lkml.kernel.org/r/20241213032244.877029-1-vineeth@bitbyteword.org

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