netfilter: ipset: Make struct htype per ipset family

Before this patch struct htype created at the first source
of ip_set_hash_gen.h and it is common for both IPv4 and IPv6
set variants.

Make struct htype per ipset family and use NLEN to make
nets array fixed size to simplify struct htype allocation.

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

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>