i40e: implement flush flag for ndo_xdp_xmit

When passed the XDP_XMIT_FLUSH flag i40e_xdp_xmit now performs the
same kind of ring tail update as in i40e_xdp_flush.  The advantage is
that all the necessary checks have been performed and xdp_ring can be
updated, instead of having to perform the exact same steps/checks in
i40e_xdp_flush

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

drivers/net/ethernet/intel/i40e/i40e_txrx.c

@@ -3676,6 +3676,7 @@ int i40e_xdp_xmit(struct net_device *dev, int n, struct xdp_frame **frames,
 	struct i40e_netdev_priv *np = netdev_priv(dev);
 	unsigned int queue_index = smp_processor_id();
 	struct i40e_vsi *vsi = np->vsi;
+	struct i40e_ring *xdp_ring;
 	int drops = 0;
 	int i;
 
@@ -3685,20 +3686,25 @@ int i40e_xdp_xmit(struct net_device *dev, int n, struct xdp_frame **frames,
 	if (!i40e_enabled_xdp_vsi(vsi) || queue_index >= vsi->num_queue_pairs)
 		return -ENXIO;
 
-	if (unlikely(flags & ~XDP_XMIT_FLAGS_NONE))
+	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK))
 		return -EINVAL;
 
+	xdp_ring = vsi->xdp_rings[queue_index];
+
 	for (i = 0; i < n; i++) {
 		struct xdp_frame *xdpf = frames[i];
 		int err;
 
-		err = i40e_xmit_xdp_ring(xdpf, vsi->xdp_rings[queue_index]);
+		err = i40e_xmit_xdp_ring(xdpf, xdp_ring);
 		if (err != I40E_XDP_TX) {
 			xdp_return_frame_rx_napi(xdpf);
 			drops++;
 		}
 	}
 
+	if (unlikely(flags & XDP_XMIT_FLUSH))
+		i40e_xdp_ring_update_tail(xdp_ring);
+
 	return n - drops;
 }