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>
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 9e28768..b44d7b7 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -728,6 +728,7 @@
 
 drop:
 	dev->stats.tx_dropped++;
+	skb_tx_error(skb);
 	kfree_skb(skb);
 	rcu_read_unlock();
 	return NETDEV_TX_OK;