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

*** 4148,4158 ****
          list_remove(&kmem_caches, cp);
          mutex_exit(&kmem_cache_lock);
  
          if (kmem_taskq != NULL)
                  taskq_wait(kmem_taskq);
!         if (kmem_move_taskq != NULL)
                  taskq_wait(kmem_move_taskq);
  
          kmem_cache_magazine_purge(cp);
  
          mutex_enter(&cp->cache_lock);
--- 4148,4159 ----
          list_remove(&kmem_caches, cp);
          mutex_exit(&kmem_cache_lock);
  
          if (kmem_taskq != NULL)
                  taskq_wait(kmem_taskq);
! 
!         if (kmem_move_taskq != NULL && cp->cache_defrag != NULL)
                  taskq_wait(kmem_move_taskq);
  
          kmem_cache_magazine_purge(cp);
  
          mutex_enter(&cp->cache_lock);