netfilter: ipset: Mark some helper args as const.

Mark some of the helpers arguments as const.

Ported from a patch proposed by Sergey Popovich <popovich_sergei@mail.ua>.

Suggested-by: Sergey Popovich <popovich_sergei@mail.ua>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>