net/mlx5e: XDP, Remove un-established assumptions on XDP buffer

Remove the assumption of non-zero linear length in the XDP xmit
function, used to serve both internal XDP_TX operations as well as
redirected-in requests.

Do not apply the MLX5E_XDP_MIN_INLINE check unless necessary.

Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>