4730 Destroy the metaslab group taskq in metaslab_group_destroy()

*** 345,354 ****
--- 345,356 ----
           * either because we never activated in the first place or
           * because we're done, and possibly removing the vdev.
           */
          ASSERT(mg->mg_activation_count <= 0);
  
+         if (mg->mg_taskq) taskq_destroy(mg->mg_taskq);
+ 
          avl_destroy(&mg->mg_metaslab_tree);
          mutex_destroy(&mg->mg_lock);
          kmem_free(mg, sizeof (metaslab_group_t));
  }