perf_events, trace: Fix perf_trace_destroy(), mutex went missing

Steve spotted I forgot to do the destroy under event_mutex.
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1274451913.1674.1707.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_event_perf.c

@@ -132,8 +132,9 @@ void perf_trace_destroy(struct perf_event *p_event)
 	struct ftrace_event_call *tp_event = p_event->tp_event;
 	int i;
 
+	mutex_lock(&event_mutex);
 	if (--tp_event->perf_refcount > 0)
-		return;
+		goto out;
 
 	if (tp_event->class->reg)
 		tp_event->class->reg(tp_event, TRACE_REG_PERF_UNREGISTER);
@@ -157,6 +158,8 @@ void perf_trace_destroy(struct perf_event *p_event)
 			perf_trace_buf[i] = NULL;
 		}
 	}
+out:
+	mutex_unlock(&event_mutex);
 }
 
 __kprobes void *perf_trace_buf_prepare(int size, unsigned short type,