OS-4162 system appears (mostly) hung on kmem_move taskq

@@ -4148,11 +4148,12 @@
         list_remove(&kmem_caches, cp);
         mutex_exit(&kmem_cache_lock);
 
         if (kmem_taskq != NULL)
                 taskq_wait(kmem_taskq);
-        if (kmem_move_taskq != NULL)
+
+        if (kmem_move_taskq != NULL && cp->cache_defrag != NULL)
                 taskq_wait(kmem_move_taskq);
 
         kmem_cache_magazine_purge(cp);
 
         mutex_enter(&cp->cache_lock);