tun: report orphan frags errors to zero copy callback

When tun transmits a zero copy skb, it orphans the frags
which might need to allocate extra memory, in atomic context.
If that fails, notify ubufs callback before freeing the skb
as a hint that device should disable zerocopy mode.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -728,6 +728,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 
 drop:
 	dev->stats.tx_dropped++;
+	skb_tx_error(skb);
 	kfree_skb(skb);
 	rcu_read_unlock();
 	return NETDEV_TX_OK;