netfilter: add -n to iptables and ip6tables calls

To preload netfilter modules, criu runs "iptables -L" and "ip6tables -L"
before starting to dump or restore a process tree.

On systems with many entries, the above commands without the -n option
take a long time because of lengthy DNS lookups.

Signed-off-by: Saied Kazemi <saied@google.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>