netfilter: flowtable: add nf_flowtable_time_stamp

This patch adds nf_flowtable_time_stamp and updates the existing code to
use it.

This patch is also implicitly fixing up hardware statistic fetching via
nf_flow_offload_stats() where casting to u32 is missing. Use
nf_flow_timeout_delta() to fix this.

Fixes: c29f74e0

 ("netfilter: nf_flow_table: hardware offload support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: wenxu <wenxu@ucloud.cn>