net/mlx5e: Add support for UDP tunnel segmentation with outer checksum offload

This patch assumes that the mlx5 hardware will ignore existing IPv4/v6
header fields for length and checksum as well as the length and checksum
fields for outer UDP headers.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>