list_ip_routes should only fetch needed devices

[Adam Oswick]

Summary
-------
The list_ip_routes function calls privileged.get_link_devices to fetch all devices and then filters out just the ones that it actually wants.

Description
-----------
Fetching all devices, particularly in larger network namespaces, is slow. This seems to be a result of the (de)serialisation of data being sent between the privileged privsep process and the normal proc.

The list_ip_routes fetches all of these devices from the privileged process and then filters the result in the unprivileged process. However, privileged.get_link_devices accepts an argument for the interface index (RTA_OIF) allowing us to just ask the privileged process for the device we are actually interested in.

Pre-conditions
--------------
Nothing specific. This function always appears to this.

Step by step reproduction steps
-------------------------------
N/A

Expected output
---------------
The list_ip_routes function completes quickly and only fetches necessary data from the privsep process.

Actual output
-------------
The list_ip_routes function completes slowly (relatively speaking) and fetches excessive information from the privsep process, most of which is then discarded.

Version
-------
OpenStack Antelope

[Brian Haley (brian-haley)]

https://review.opendev.org/c/openstack/neutron/+/900415 is the proposed change.