MDEV-28920 Rescheduling of innodb_stats_func() missing

Fixed tpool timer implementation on POSIX.
Prior to this patch, under some specific rare circumstances (concurrency
related), timer callback execution might be skipped.

tpool/tpool_generic.cc

@@ -345,22 +345,23 @@ class thread_pool_generic : public thread_pool
     int m_period;
     std::mutex m_mtx;
     bool m_on;
-    std::atomic<bool> m_running;
+    std::atomic<int> m_running;
 
     void run()
     {
       /*
         In rare cases, multiple callbacks can be scheduled,
-        e.g with set_time(0,0) in a loop.
-        We do not allow parallel execution, as user is not prepared.
+        at the same time,. e.g with set_time(0,0) in a loop.
+        We do not allow parallel execution, since it is against the expectations.
       */
-      bool expected = false;
-      if (!m_running.compare_exchange_strong(expected, true))
+      if (m_running.fetch_add(1, std::memory_order_acquire) > 0)
         return;
-
+      do
+      {
         m_callback(m_data);
         dbug_execute_after_task_callback();
-      m_running = false;
+      }
+      while (m_running.fetch_sub(1, std::memory_order_release) != 1);
 
       if (m_pool && m_period)
       {