futex: Prevent inconsistent state and exit race

The recent rework of the requeue PI code introduced a possibility for
going back to user space in inconsistent state:

CPU 0				CPU 1

requeue_futex()
  if (lock_pifutex_user()) {
      dequeue_waiter();
      wake_waiter(task);
				sched_in(task);
     				return_from_futex_syscall();

  ---> Inconsistent state because PI state is not established

It becomes worse if the woken up task immediately exits:

				sys_exit();
				
      attach_pistate(vpid);	<--- FAIL


Attach the pi state before dequeuing and waking the waiter. If the waiter
gets a spurious wakeup before the dequeue operation it will wait in
futex_requeue_pi_wakeup_sync() and therefore cannot return and exit.

Fixes: 07d91ef5 ("futex: Prevent requeue_pi() lock nesting issue on RT")
Reported-by:  <syzbot+4d1bd0725ef09168e1a0@syzkaller.appspotmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210902094414.558914045@linutronix.de