netfilter: xt_addrtype: ipv6 support

The kernel will refuse certain types that do not work in ipv6 mode.
We can then add these features incrementally without risk of userspace
breakage.

Signed-off-by: Florian Westphal <fwestphal@astaro.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>