forcedeth: implement ndo_get_stats64() API

This commit implements the ndo_get_stats64() API for forcedeth. Since
hardware stats are being updated from different contexts (process and
timer), this commit adds synchronization. For software stats, it
relies on the u64_stats_sync.h API.

Tested:
  - 16-way SMP x86_64 ->
    RX bytes:7244556582 (7.2 GB)  TX bytes:181904254 (181.9 MB)
  - pktgen + loopback: identical rx_bytes/tx_bytes and rx_packets/tx_packets
Signed-off-by: David Decotigny <david.decotigny@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>