net/mlx5e: IPoIB, Add ndo stats support for IPoIB child devices

mainline inclusion
from mainline-4.20
commit b75ba382
category: bugfix
bugzilla: 6101
CVE: NA

-------------------------------------------------

Expose RX and TX counters by implementing ndo_get_stats64 operation
for child devices.

Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Keefe LIU <liuqifa@huawei.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>