net: hns3: fix for not unmapping TX buffer correctly

When a big TX buffer is sent using multi BD, the driver maps the
whole TX buffer, and unmaps it using info in desc_cb corresponding
to each BD, but only the info in the desc_cb of first BD is correct,
other info in desc_cb is wrong, which causes TX unmapping problem
when SMMU is on.

Only set the mapping and freeing info in the desc_cb of first BD to
fix this problem, because the TX buffer only need to be unmapped and
freed once.

Fixes: 1e8a7977

("net: hns3: add handling for big TX fragment")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Huzhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>