[JFR] Fixed missing Threads_lock protection when traversing thread list

Summary:
TRACE_REQUEST_FUNC(ThreadAllocationStatistics) in jfrPeriodic.cpp
void JfrCheckpointThreadClosure::do_thread(Thread* t) in jfrType.cpp

Test Plan: jdk/test/jdk/jfr

Reviewed-by: kelthuzadx, zhengxiaolinX

Issue: https://github.com/alibaba/dragonwell8/issues/134

src/share/vm/jfr/periodic/jfrPeriodic.cpp

@@ -395,6 +395,10 @@ TRACE_REQUEST_FUNC(ThreadAllocationStatistics) {
   GrowableArray<jlong> allocated(initial_size);
   GrowableArray<traceid> thread_ids(initial_size);
   JfrTicks time_stamp = JfrTicks::now();
+
+  {
+    // Collect allocation statistics while holding threads lock
+    MutexLockerEx ml(Threads_lock);
     JfrJavaThreadIterator iter;
     while (iter.has_next()) {
       JavaThread* const jt = iter.next();
@@ -402,6 +406,7 @@ TRACE_REQUEST_FUNC(ThreadAllocationStatistics) {
       allocated.append(jt->cooked_allocated_bytes());
       thread_ids.append(JFR_THREAD_ID(jt));
     }
+  }
 
   // Write allocation statistics to buffer.
   for(int i = 0; i < thread_ids.length(); i++) {

src/share/vm/jfr/recorder/checkpoint/types/jfrType.cpp

@@ -108,6 +108,7 @@ void JfrCheckpointThreadClosure::do_thread(Thread* t) {
 
 void JfrThreadConstantSet::serialize(JfrCheckpointWriter& writer) {
   JfrCheckpointThreadClosure tc(writer);
+  MutexLockerEx ml(Threads_lock);
   JfrJavaThreadIterator javathreads;
   while (javathreads.has_next()) {
     tc.do_thread(javathreads.next());