veth: introduce more specialized counters in veth_stats

Introduce xdp_tx, xdp_redirect and rx_drops counters in veth_stats data
structure. Move stats accounting in veth_poll. Remove xdp_xmit variable
in veth_xdp_rcv_one/veth_xdp_rcv_skb and rely on veth_stats counters.
This is a preliminary patch to align veth xdp statistics to mlx, intel
and marvell xdp implementation

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