netfilter: ipset: Check CIDR value only when attribute is given

There is no reason to check CIDR value regardless attribute
specifying CIDR is given.

Initialize cidr array in element structure on element structure
declaration to let more freedom to the compiler to optimize
initialization right before element structure is used.

Remove local variables cidr and cidr2 for netnet and netportnet
hashes as we do not use packed cidr value for such set types and
can store value directly in e.cidr[].

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