flow_offload: add statistics retrieval infrastructure and use it

This patch provides the flow_stats structure that acts as container for
tc_cls_flower_offload, then we can use to restore the statistics on the
existing TC actions. Hence, tcf_exts_stats_update() is not used from
drivers anymore.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>