net/mlx5: enhance error log for tunnel offloading

[ upstream commit c9fecc1cc2886bcd626a656a5493841363dec6dd ]

Tunnel offloading is linked to running a port with the non-obvious
dv_xmeta_en=3 devargs. Hint at it for "normal" users.

Fixes: 4ec6360 ("net/mlx5: implement tunnel offload")

Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Raslan Darawsheh <rasland@nvidia.com>

drivers/net/mlx5/mlx5_flow.c

@@ -9697,7 +9697,7 @@ mlx5_flow_tunnel_validate(struct rte_eth_dev *dev,
 	if (!is_tunnel_offload_active(dev))
 		return rte_flow_error_set(error, ENOTSUP,
 					  RTE_FLOW_ERROR_TYPE_ACTION_CONF, NULL,
-					  "tunnel offload was not activated");
+					  "tunnel offload was not activated, consider setting dv_xmeta_en=3");
 	if (!tunnel)
 		return rte_flow_error_set(error, EINVAL,
 					  RTE_FLOW_ERROR_TYPE_ACTION_CONF, NULL,