8261251: Shenandoah: Use object size for full GC humongous compaction

Reviewed-by: zgu
Backport-of: deb0544ff330fadc5aac0378766bcd36c220d7e2

Author: shipilev

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

@@ -915,7 +915,7 @@ void ShenandoahMarkCompact::compact_humongous_objects() {
 
       Copy::aligned_conjoint_words(heap->get_region(old_start)->bottom(),
                                    heap->get_region(new_start)->bottom(),
-                                   ShenandoahHeapRegion::region_size_words()*num_regions);
+                                   words_size);
 
       oop new_obj = oop(heap->get_region(new_start)->bottom());
       new_obj->init_mark();