4730 Destroy the metaslab group taskq in metaslab_group_destroy()

@@ -345,10 +345,12 @@
          * 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));
 }