net/mlx4_en: get rid of ret_stats

mlx4 uses a private struct net_device_stats in a vain attempt
to avoid races.

This is buggy because multiple cpus could call mlx4_en_get_stats()
at the same time, so ret_stats can not guarantee stable results.

To fix this, we need to switch to ndo_get_stats64() as this
method provides per-thread storage.

This allows to reduce mlx4_en_priv bloat.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>