lxc-net.in: fix nftables syntax for IPv6 NAT

lxc-net.in: fix nftables syntax for IPv6 NAT

The nftables masquarade rule for IPv6 was using the IPv4 syntax. This
resulted in the following error when starting the lxc-net.service with
LXC_IPV6_NAT="true" and nftables:

    Feb 11 18:54:54 pc lxc-net[4936]: Error: conflicting protocols specified: ip6 vs. ip
    Feb 11 18:54:54 pc lxc-net[4936]:                              ^^^^^^^^
    Feb 11 18:54:54 pc lxc-net[4917]: Failed to setup lxc-net.
    Feb 11 18:54:54 pc systemd[1]: lxc-net.service: Main process exited, code=exited, status=1/FAILURE
    Feb 11 18:54:54 pc systemd[1]: lxc-net.service: Failed with result 'exit-code'.
    Feb 11 18:54:54 pc systemd[1]: Failed to start LXC network bridge setup.

Signed-off-by: Quentin Lyons <36303164+n0p90@users.noreply.github.com>