net: hns3: check the return of skb_checksum_help()

Currently skb_checksum_help()'s return is ignored, but it may
return error when it fails to allocate memory when linearizing.

So adds checking for the return of skb_checksum_help().

Fixes: 76ad4f0e("net: hns3: Add support of HNS3 Ethernet Driver for hip08 SoC")
Fixes: 3db084d2("net: hns3: Fix for vxlan tx checksum bug")
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>