6639341: sometimes contended-exit event comes after contended-entered on another thread

Summary: DTrace probe "contended-exit" should be fired before unparking object, or context could be lost. Probe firing was moved to proper place.
Reviewed-by: coleenp, kamg

src/share/vm/runtime/synchronizer.cpp

@@ -3363,13 +3363,13 @@ void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
    //   If the wakee is cold then transiently setting it's affinity
    //   to the current CPU is a good idea.
    //   See http://j2se.east/~dice/PERSIST/050624-PullAffinity.txt
+   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
    Trigger->unpark() ;
 
    // Maintain stats and report events to JVMTI
    if (ObjectSynchronizer::_sync_Parks != NULL) {
       ObjectSynchronizer::_sync_Parks->inc() ;
    }
-   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
 }