8247670: Shenandoah: deadlock during class unloading OOME

Reviewed-by: rkennke

src/hotspot/share/gc/shenandoah/shenandoahCodeRoots.cpp

@@ -273,6 +273,7 @@ public:
 
     // Heal oops and disarm
     if (_bs->is_armed(nm)) {
+      ShenandoahEvacOOMScope oom_evac_scope;
       ShenandoahNMethod::heal_nmethod_metadata(nm_data);
       _bs->disarm(nm);
     }
@@ -311,7 +312,6 @@ public:
 
   virtual void work(uint worker_id) {
     ICRefillVerifierMark mark(_verifier);
-    ShenandoahEvacOOMScope evac_scope;
     _iterator.nmethods_do(&_cl);
   }