ipvs: use common functions for stats allocation

Move alloc_percpu/free_percpu logic in new functions

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>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>