sched/completion: Fix a lockup in wait_for_completion()

Consider following race:

  T0                    T1                       T2
  wait_for_completion()
   do_wait_for_common()
    __prepare_to_swait()
     schedule()
                        complete()
                         x->done++ (0 -> 1)
                         raw_spin_lock_irqsave()
                         swake_up_locked()       wait_for_completion()
                          wake_up_process(T0)
                          list_del_init()
                         raw_spin_unlock_irqrestore()
                                                  raw_spin_lock_irq(&x->wait.lock)
  raw_spin_lock_irq(&x->wait.lock)                x->done != UINT_MAX, 1 -> 0
                                                  raw_spin_unlock_irq(&x->wait.lock)
                                                  return 1
   while (!x->done && timeout),
   continue loop, not enqueued
   on &x->wait

Basically, the problem is that the original wait queues used in
completions did not remove the item from the queue in the wakeup
function, but swake_up_locked() does.

Fix it by adding the thread to the wait queue inside the do loop.
The design of swait detects if it is already in the list and doesn't
do the list add again.

Cc: stable-rt@vger.kernel.org
Fixes: a04ff6b4ec4ee7e ("completion: Use simple wait queues")
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
[bigeasy: shorten commit message ]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>