Description of problem:

In bz 2099811 it was reported that frames queued for userspace upcall were being reported as dropped in queue_userspace_packet(), even in cases where there was no error.

> 560 ▹ err = genlmsg_unicast(ovs_dp_get_net(dp), user_skb,
> upcall_info->portid);↩ <---- this returns 0
> 561 ▹ user_skb = NULL;↩
> 562 out:↩
> 563 ▹ if (err)↩
> 564 ▹ ▹ skb_tx_error(skb);↩
> 565 ▹ kfree_skb(user_skb);↩
> 566 ▹ kfree_skb(nskb);↩ <---- and it call kfree_skb() instead of
> __kfree_skb() here incorrectly (this is a minor bug)
> 567 ▹ return err;↩
> 568 }↩