netfilter: ipset: Make NLEN compile time constant for hash types

Hash types define HOST_MASK before inclusion of ip_set_hash_gen.h
and the only place where NLEN needed to be calculated at runtime
is *_create() method.

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

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