Commit 4f47013feb introduced a new validation step to make sure no
IPv6 subnet is configured on a network which has EnableIPv6=false.
Commit 5d5eeac310 then removed that validation step and automatically
enabled IPv6 for networks with a v6 subnet. But this specific commit
was reverted in c59e93a67b and now the error introduced by 4f47013feb
is re-introduced.
But it turns out some users expect a network created with an IPv6
subnet and EnableIPv6=false to actually have no IPv6 connectivity.
This restores that behavior.
Signed-off-by: Albin Kerouanton <albinker@gmail.com>
e37172c613