To properly implement 64bits network statistics on 32bit or 64bit hosts,
we provide one new type and four methods, to ease conversions.

Stats producer should use following template granted it already got an
exclusive access to counters (include/linux/u64_stats_sync.h contains
some documentation about details)

    u64_stats_update_begin(&stats->syncp);
    stats->bytes64 += len;
    stats->packets64++;
    u64_stats_update_end(&stats->syncp);

While a consumer should use following template to get consistent
snapshot :

    u64 tbytes, tpackets;
    unsigned int start;

    do {
        start = u64_stats_fetch_begin(&stats->syncp);
        tbytes = stats->bytes64;
        tpackets = stats->packets64;
    } while (u64_stats_fetch_retry(&stats->lock, syncp));

Suggested by David Miller, and comments courtesy of Nick Piggin.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>