When ping is runnig and user executes the loopback selftest, the
ping cmd will stop and exit.

This patch fixes it by using the hns3_nic_net_open/stop to offline
the netdev when doing loopback selftest.

Fixes: c39c4d98 ("net: hns3: Add mac loopback selftest support in hns3 driver")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>