netfilter: nf_tables: 64bit stats need some extra synchronization

Use generic u64_stats_sync infrastructure to get proper 64bit stats,
even on 32bit arches, at no extra cost for 64bit arches.

Without this fix, 32bit arches can have some wrong counters at the time
the carry is propagated into upper word.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>