netfilter: ipset: Separate memsize calculation code into dedicated function

Hash types already has it's memsize calculation code in separate
functions. Clean up and do the same for *bitmap* and *list* sets.

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>