netfilter: ipset: hash:net set type support

The module implements the hash:net type support in four flavours:
for IPv4 and IPv6, both without and with timeout support. The elements
are one dimensional: IPv4/IPv6 network address/prefixes.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>