Some init_router_port steps are unnecessary for new devices

[Adam Oswick]

Summary
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When doing the init for new router ports, some of the steps are not needed and just add extra overhead.

Description
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These unnecessary steps slow down the router setup. If the entire network is being setup from scratch, this can increase the time it takes for VMs to become reachable.

Pre-conditions
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The Neutron L3 agent is attempting to setup a router for the first time or from scratch (e.g. following a complete removal of the qrouter & fip namespaces). For replication purposes, this should be a router with a large number of subnets attached (~100).

Step by step reproduction steps
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On a compute node with DVR:

ip netns delete $QROUTER_NETNS && ip netns delete $FIP_NETNS && docker restart neutron_l3_agent

Note: How the agent is restarted will of course depend on how OpenStack is being deployed.

Expected output
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Unnecessary steps for new interfaces are skipped.

Actual output
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Unnecessary steps for new interfaces are carried out but result in no changes and delay the router setup being completed.

Version
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OpenStack Antelope

[Jakub Libosvar (libosvar)]

Related proposed patch: https://review.opendev.org/c/openstack/neutron/+/900553