veth: remove atomic64_add from veth_xdp_xmit hotpath

Remove atomic64_add from veth_xdp_xmit hotpath and rely on
xdp_xmit_err/xdp_tx_err counters

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>