network: unset Link.ndisc_configured only when a new address or route is requested

network: unset Link.ndisc_configured only when a new address or route is requested

This fixes an issue introduced by af2aea8bb64b0dc42ecbe5549216eb567681a803.

When an outdated address or route is passed to link_request_address()/route(),
then they return 0 and the address or route will not be assigned. Such
situation can happen when we receive RA with zero lifetime. In that
case, we should not unset Link.ndisc_configured flag, otherwise even
no new address nor route will assigned, the interface will enter to the
configuring state, and unnecessary DBus property change is emit and the state
file will be updated. That makes resolved or timesyncd triggered to
reconfigure the interface.

Fixes #25456.