netfilter: ipset: Fix hashing for ipv6 sets

HKEY_DATALEN remains defined after first inclusion
of ip_set_hash_gen.h, so it is incorrectly reused
for IPv6 code.

Undefine HKEY_DATALEN in ip_set_hash_gen.h at the end.

Also remove some useless defines of HKEY_DATALEN in
ip_set_hash_{ip{,mark,port},netiface}.c as ip_set_hash_gen.h
defines it correctly for such set types anyway.

Signed-off-by: Sergey Popovich <popovich_sergei@mail.ua>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>