netfilter: ipset: Permit CIDR equal to the host address CIDR in IPv6

Permit userspace to supply CIDR length equal to the host address CIDR
length in netlink message. Prohibit any other CIDR length for IPv6
variant of the set.

Also return -IPSET_ERR_HASH_RANGE_UNSUPPORTED instead of generic
-IPSET_ERR_PROTOCOL in IPv6 variant of hash:ip,port,net when
IPSET_ATTR_IP_TO attribute is given.

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