8255848: -Xlog:gc+heap+exit shows "used 0K"

Reviewed-by: tschatzl, ayang

Author: kstefanj

src/hotspot/share/gc/g1/g1CollectedHeap.cpp

@@ -2437,10 +2437,11 @@ void G1CollectedHeap::print_heap_regions() const {
 }
 
 void G1CollectedHeap::print_on(outputStream* st) const {
+  size_t heap_used = Heap_lock->owned_by_self() ? used() : used_unlocked();
   st->print(" %-20s", "garbage-first heap");
   if (_hrm != NULL) {
     st->print(" total " SIZE_FORMAT "K, used " SIZE_FORMAT "K",
-              capacity()/K, used_unlocked()/K);
+              capacity()/K, heap_used/K);
     st->print(" [" PTR_FORMAT ", " PTR_FORMAT ")",
               p2i(_hrm->reserved().start()),
               p2i(_hrm->reserved().end()));