MDEV-26043: Fix performance_schema instrument "threadpool/group_mutex"

The performance_schema data related to instrument "wait/synch/mutex/threadpool/group_mutex" was incorrect. The root cause is the group_mutex was initialized in thread_group_init before registerd in PSI_register(mutex).
The fix is to put PSI_register before thread_group_init, which ensures the right order.

sql/threadpool_generic.cc

@@ -1601,6 +1601,9 @@ int TP_pool_generic::init()
     sql_print_error("Allocation failed");
     DBUG_RETURN(-1);
   }
+  PSI_register(mutex);
+  PSI_register(cond);
+  PSI_register(thread);
   scheduler_init();
   threadpool_started= true;
   for (uint i= 0; i < threadpool_max_size; i++)
@@ -1614,10 +1617,6 @@ int TP_pool_generic::init()
     sql_print_error("Can't set threadpool size to %d",threadpool_size);
     DBUG_RETURN(-1);
   }
-  PSI_register(mutex);
-  PSI_register(cond);
-  PSI_register(thread);
-
   pool_timer.tick_interval= threadpool_stall_limit;
   start_timer(&pool_timer);
   DBUG_RETURN(0);