6810698: G1: two small bugs in the sparse remembered sets

Summary: The _expanded flag of the sparse RSets is not reset and this can leave a RSet in an inconsistent state if it is expanded more than once. Also, we should be iterating over the _cur, instead of the _next, sparse table
Reviewed-by: apetrusenko, iveresov

src/share/vm/gc_implementation/g1/sparsePRT.cpp

@@ -504,6 +504,7 @@ void SparsePRT::cleanup() {
   // Make sure that the current and next tables agree.  (Another mechanism
   // takes care of deleting now-unused tables.)
   _cur = _next;
+  set_expanded(false);
 }
 
 void SparsePRT::expand() {

src/share/vm/gc_implementation/g1/sparsePRT.hpp

@@ -274,7 +274,7 @@ public:
 
   // Clean up all tables on the expanded list.  Called single threaded.
   static void cleanup_all();
-  RSHashTable* next() const { return _next; }
+  RSHashTable* cur() const { return _cur; }
 
 
   void init_iterator(SparsePRTIter* sprt_iter);
@@ -300,7 +300,7 @@ public:
   {}
 
   void init(const SparsePRT* sprt) {
-    RSHashTableIter::init(sprt->next());
+    RSHashTableIter::init(sprt->cur());
   }
   bool has_next(size_t& card_index) {
     return RSHashTableIter::has_next(card_index);