net/i40e: fix forward outer IPv6 VXLAN

net/i40e: fix forward outer IPv6 VXLAN

Testpmd forwards packets in checksum mode that it need to calculate
the checksum of each layer's protocol. Then it will fill flags and
header length into mbuf.

In process_outer_cksums, HW calculates the outer checksum if
tx_offloads contains outer UDP checksum otherwise SW calculates
the outer checksum.

When tx_offloads contains outer UDP checksum or outer IPv4 checksum,
mbuf will be filled with correct header length.

This patch added outer UDP checksum in tx_offload_capa and
I40E_TX_OFFLOAD_MASK, when we set csum hw outer-udp on that the
engine can forward outer IPv6 VXLAN packets.

Fixes: 7497d3e2f777 ("net/i40e: convert to new Tx offloads API")
Cc: stable@dpdk.org
Signed-off-by: Jie Wang <jie1x.wang@intel.com>
Acked-by: Beilei Xing <beilei.xing@intel.com>