perf_events: Update the context time on exit

It appeared we did call update_event_times() on exit, but we
failed to update the context time, which renders the former
moot.

Locking is a bit iffy, we call update_event_times under
ctx->mutex instead of ctx->lock - the next patch fixes this.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20091123103819.764207355@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -4983,6 +4983,7 @@ void perf_event_exit_task(struct task_struct *child)
 	 * the events from it.
 	 */
 	unclone_ctx(child_ctx);
+	update_context_time(child_ctx);
 	spin_unlock_irqrestore(&child_ctx->lock, flags);
 
 	/*