sched: Do not call __put_task_struct() on rt if pi_blocked_on is set

With PREEMPT_RT enabled, some of the calls to put_task_struct() coming
from rt_mutex_adjust_prio_chain() could happen in preemptible context and
with a mutex enqueued. That could lead to this sequence:

        rt_mutex_adjust_prio_chain()
          put_task_struct()
            __put_task_struct()
              sched_ext_free()
                spin_lock_irqsave()
                  rtlock_lock() --->  TRIGGERS
                                      lockdep_assert(!current->pi_blocked_on);

This is not a SCHED_EXT bug. The first cleanup function called by
__put_task_struct() is sched_ext_free() and it happens to take a
(RT) spin_lock, which in the scenario described above, would trigger
the lockdep assertion of "!current->pi_blocked_on".

Crystal Wood was able to identify the problem as __put_task_struct()
being called during rt_mutex_adjust_prio_chain(), in the context of
a process with a mutex enqueued.

Instead of adding more complex conditions to decide when to directly
call __put_task_struct() and when to defer the call, unconditionally
resort to the deferred call on PREEMPT_RT to simplify the code.

Fixes: 893cdaaa ("sched: avoid false lockdep splat in put_task_struct()")
Suggested-by: Crystal Wood <crwood@redhat.com>
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Wander Lairson Costa <wander@redhat.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/aGvTz5VaPFyj0pBV@uudg.org