net/mlx5: zero out UDP checksum in encapsulation

net/mlx5: zero out UDP checksum in encapsulation

Mellanox NICs do not support UDP checksum hardware tx offload over IPv6.
This limitation becomes critical for UDP based tunnels like VXLAN.
Beside the UDP checksum validity is required by IPv6 there is an option
in Linux to allow accepting UDP zero sum (see udp6zerocsumrx in iproute2
package).

This patch zeroes out the UDP checksum field for encapsulation headers
in raw encap action.

Signed-off-by: Eli Britstein <elibr@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>