network: IPv6 Compliance: Router Advertisement Processing, Reachable Time [v6LC.2...

network: IPv6 Compliance: Router Advertisement Processing, Reachable Time [v6LC.2.2.15] (#32792)

Previously, RA option fields were being ignored when the Router Lifetime
value was zero. Remove this logic to be compliant with RFC4861.

Extract from: https://www.ietf.org/rfc/rfc4861.html#section-4.2, p.21,
first paragraph:

    The Router Lifetime applies only to
    the router's usefulness as a default router; it
    does not apply to information contained in other
    message fields or options.

This affected IPv6 Conformance test:
    v6LC.2.2.15: Router Advertisement Processing, Reachable Time.

Fixes #31842.

Co-authored-by: Matt Muggeridge <Matt.Muggeridge@hpe.com>