When an event with pending waiters is destroyed, those waiters may
end up sleeping forever unless they are notified and woken up.
Implement the notification by clearing the waiter->event pointer,
which becomes invalid anyway, when the event is freed, and waking
up the waiting tasks.

Waiters on an event that's destroyed return failure.
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>