bridge: 64bit rx/tx counters

Use u64_stats_sync infrastructure to provide 64bit rx/tx
counters even on 32bit hosts.

It is safe to use a single u64_stats_sync for rx and tx,
because BH is disabled on both, and we use per_cpu data.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>