MDEV-17441 - InnoDB transition to C++11 atomics

trx_t::n_ref transition to Atomic_counter.

storage/innobase/include/trx0trx.h

@@ -773,7 +773,7 @@ struct trx_t {
     that it is no longer "active".
   */
 
-  int32_t n_ref;
+  Atomic_counter<int32_t> n_ref;
 
 
 public:
@@ -1119,16 +1119,16 @@ struct trx_t {
 
   bool is_referenced()
   {
-    return my_atomic_load32_explicit(&n_ref, MY_MEMORY_ORDER_RELAXED) > 0;
+    return n_ref > 0;
   }
 
 
   void reference()
   {
 #ifdef UNIV_DEBUG
-  int32_t old_n_ref=
+    auto old_n_ref=
 #endif
-    my_atomic_add32_explicit(&n_ref, 1, MY_MEMORY_ORDER_RELAXED);
+    n_ref++;
     ut_ad(old_n_ref >= 0);
   }
 
@@ -1136,9 +1136,9 @@ struct trx_t {
   void release_reference()
   {
 #ifdef UNIV_DEBUG
-  int32_t old_n_ref=
+    auto old_n_ref=
 #endif
-    my_atomic_add32_explicit(&n_ref, -1, MY_MEMORY_ORDER_RELAXED);
+    n_ref--;
     ut_ad(old_n_ref > 0);
   }