net: u64_stats_sync improvements

 *    pure reads. But if they have to fetch many values, it's better to not allow

 *    preemptions/interruptions to avoid many retries.

 *

 * 6) If counter might be written by an interrupt, readers should block interrupts.

 *    (On UP, there is no seqcount_t protection, a reader allowing interrupts could

 *     read partial values)

 *

 * Usage :

 *

 * Stats producer (writer) should use following template granted it already got

 *         start = u64_stats_fetch_begin(&stats->syncp);

 *         tbytes = stats->bytes64; // non atomic operation

 *         tpackets = stats->packets64; // non atomic operation

 * } while (u64_stats_fetch_retry(&stats->lock, syncp));

 * } while (u64_stats_fetch_retry(&stats->syncp, start));

 *

 *

 * Example of use in drivers/net/loopback.c, using per_cpu containers,