net/hns3: fix Tx checksum with fixed header length

net/hns3: fix Tx checksum with fixed header length

Currently, the header length of all the layers are fixed, It would
lead to a csum error when the header length changed.

This patch fixes above problem by using the header length in mbuf
instead of the fixed header length to perform the TX cksum offload.

Fixes: bba636698316 ("net/hns3: support Rx/Tx and related operations")
Cc: stable@dpdk.org
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Lijun Ou <oulijun@huawei.com>