net: hns: fix return value check in __lb_other_process()

The function skb_copy() could return NULL, the return value
need to be checked.

Fixes: b5996f11 ("net: add Hisilicon Network Subsystem basic ethernet support")
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>