8318720: G1: Memory leak in G1CodeRootSet after JDK-8315503

Reviewed-by: tschatzl

src/hotspot/share/gc/g1/g1CodeRootSet.cpp

@@ -143,8 +143,11 @@ public:
   }
 
   void clear() {
-    _table.unsafe_reset();
-    Atomic::store(&_num_entries, (size_t)0);
+    // Remove all entries.
+    auto always_true = [] (nmethod** value) {
+                         return true;
+                       };
+    clean(always_true);
   }
 
   void iterate_at_safepoint(CodeBlobClosure* blk) {