Unlock mutex before going back to waiting for PI mutexes

[BZ #14417]
A futex call with FUTEX_WAIT_REQUEUE_PI returns with the mutex locked
on success.  If such a successful thread is pipped to the cond_lock by
another spuriously woken waiter, it could be sent back to wait on the
futex with the mutex lock held, thus causing a deadlock.  So it is
necessary that the thread relinquishes the mutex before going back to
sleep.