OCPBUGS-41785: Validate MTU when controlPlanePort uses FixedIPs

The minimum MTU for the IPv6 network was previously only enforced when
the controlPlanePort used the Network filter. This commit change the
validation to also work when using the Subnet filter from FixedIPs.

This commit does two things:
- populate the ControlPlanePortNetwork struct in CloudInfo even when
  using the Subnet filter and not the Network one.
- move the validation for MTU out of the branch for Network filter from
  the install-config.yaml

pkg/asset/installconfig/openstack/validation/cloudinfo.go

@@ -380,11 +380,15 @@ func (ci *CloudInfo) getNetwork(ctx context.Context, controlPlanePort *openstack
 	networkName := controlPlanePort.Network.Name
 	networkID := controlPlanePort.Network.ID
 	if networkName == "" && networkID == "" {
-		return nil, nil
+		if len(ci.ControlPlanePortSubnets) > 0 && ci.ControlPlanePortSubnets[0].NetworkID != "" {
+			networkID = ci.ControlPlanePortSubnets[0].NetworkID
+		} else {
+			return nil, nil
+		}
 	}
 	opts := networks.ListOpts{}
 	if networkID != "" {
-		opts.ID = controlPlanePort.Network.ID
+		opts.ID = networkID
 	}
 	if networkName != "" {
 		opts.Name = controlPlanePort.Network.Name

pkg/asset/installconfig/openstack/validation/platform.go

@@ -97,9 +97,9 @@ func validateControlPlanePort(p *openstack.Platform, n *types.Networking, ci *Cl
 		} else if ci.ControlPlanePortNetwork.ID != networkID {
 			allErrs = append(allErrs, field.Invalid(fldPath.Child("controlPlanePort").Child("network"), networkDetail, "network must contain subnets"))
 		}
-		if hasIPv6Subnet && ci.ControlPlanePortNetwork.MTU < minimumMTUv6 {
-			allErrs = append(allErrs, field.InternalError(fldPath.Child("controlPlanePort").Child("network"), fmt.Errorf("network should have an MTU of at least %d", minimumMTUv6)))
-		}
+	}
+	if hasIPv6Subnet && ci.ControlPlanePortNetwork != nil && ci.ControlPlanePortNetwork.MTU < minimumMTUv6 {
+		allErrs = append(allErrs, field.InternalError(fldPath.Child("controlPlanePort").Child("network"), fmt.Errorf("network should have an MTU of at least %d", minimumMTUv6)))
 	}
 	return allErrs
 }