portmapper: touch iptables only for IPv4
portmapper: touch iptables only for IPv4

split out of https://github.com/docker/docker/pull/20315
in order to fix https://github.com/docker/docker/issues/11518

Signed-off-by: Michael Stapelberg <97e0203ad32d728c266b6422d4bd97c18b9b600f@google.com>

medallia/libnetwork,ityangchen/libnetwork,docker/libnetwork,vdemeester/libnetwork,medallia/libnetwork,sainath14/libnetwork,mahak/libnetwork,curtiszimmerman/libnetwork,fcrisciani/libnetwork,WeiZhang555/libnetwork,mahak/libnetwork,vdemeester/libnetwork,mbanikazemi/libnetwork,tmakita/libnetwork,mYmNeo/libnetwork,allencloud/libnetwork,abhi/libnetwork,mbanikazemi/libnetwork,ityangchen/libnetwork,abhi/libnetwork,vdemeester/libnetwork,medallia/libnetwork,allencloud/libnetwork,mahak/libnetwork,curtiszimmerman/libnetwork,mYmNeo/libnetwork,fcrisciani/libnetwork,docker/libnetwork,kolyshkin/libnetwork,kolyshkin/libnetwork,kolyshkin/libnetwork,docker/libnetwork,WeiZhang555/libnetwork,msabansal/libnetwork,cpuguy83/libnetwork,msabansal/libnetwork,tmakita/libnetwork,cpuguy83/libnetwork,abhi/libnetwork,sainath14/libnetwork