8327501: Common ForkJoinPool prevents class unloading in some cases

Reviewed-by: alanb

Author: Viktor Klang

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java

@@ -1140,9 +1140,12 @@ public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
             boolean isCommon = (pool.workerNamePrefix == null);
             @SuppressWarnings("removal")
             SecurityManager sm = System.getSecurityManager();
-            if (sm == null)
-                return new ForkJoinWorkerThread(null, pool, true, false);
-            else if (isCommon)
+            if (sm == null) {
+                if (isCommon)
+                    return new ForkJoinWorkerThread.InnocuousForkJoinWorkerThread(pool);
+                else
+                    return new ForkJoinWorkerThread(null, pool, true, false);
+            } else if (isCommon)
                 return newCommonWithACC(pool);
             else
                 return newRegularWithACC(pool);