revisit ObjectMonitor padding between fields

[~rehn] mentioned this RFE during the JDK-8253064 review:

 src/hotspot/share/runtime/objectMonitor.hpp
  DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE, sizeof(volatile markWord) +
                        sizeof(WeakHandle) + sizeof(AllocationState));
                        sizeof(WeakHandle));
  // Used by async deflation as a marker in the _owner field:


I have test with and without padding, I saw no difference.