Creating subnet for exsiting external network is time-comsuming

[LIU Yulong]

If an external network has many router on it, then create a subnet will be time-consuming. Neutron will try to update all routers' external gateway ports anyway, ignore the subnet service_type.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/neutron/+/901225

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/neutron/+/901225
Committed: https://opendev.org/openstack/neutron/commit/20dbc47fc65f6ff33fa730fcb8ba02e45b8cab11
Submitter: "Zuul (22348)"
Branch: master

commit 20dbc47fc65f6ff33fa730fcb8ba02e45b8cab11
Author: LIU Yulong <email address hidden>
Date: Fri Nov 17 12:01:19 2023 +0800

    Check subnet service_type for subnet_create

    If create subnet for existing external network, check the service_type
    to skip all exsiting router gateway ports updating. And this check is
    added only for L3RouterPlugin.

    IPv6 subnet creation for external network will keep the current behavior.
    IPv6 subnet has not 'floatingip' types. And IPv6 subnet should be created
    once enough, because a /64 CIDR is large enough typically. Creating IPv6
    subnets many times does not occur frequently. So make IPv6 run as it is.

    Closes-Bug: #2043761
    Change-Id: Ica03d091572025fd6938ad1fdf4b13854625f267