ipvs: use u64_stats_t for the per-cpu counters

Use the provided u64_stats_t type to avoid
load/store tearing.

Fixes: 316580b6 ("u64_stats: provide u64_stats_t type")
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Cc: yunhong-cgl jiang <xintian1976@gmail.com>
Cc: "dust.li" <dust.li@linux.alibaba.com>
Reviewed-by: Jiri Wiesner <jwiesner@suse.de>
Tested-by: Jiri Wiesner <jwiesner@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>