net: hns3: combine len and checksum handling for inner and outer header.

When filling len and checksum info to description, there is some
similar checking or calculation.

So this patch adds hns3_set_l2l3l4 to fill the inner(/normal)
header's len and checksum info. If it is a encapsulation skb, it
calls hns3_set_outer_l2l3l4 to handle the outer header's len and
checksum info, in order to avoid some similar checking or
calculation.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>