network/ndisc: check if there exists a conflicting address

network/ndisc: check if there exists a conflicting address

Follow-up for 0a0c2672dbd22dc85d660e5baa7e1bef701beb88.

Before the commit, if a conflicting address exists or already requested,
then the configuration of newly requested address (especially, prefix
length) is mostly ignored silently.

However, after the commit, even if there exists a conflicting address,
networkd anyway tries to configure the newly requested address, and
enter failed state. Such situation can be triggered, e.g. when the DHCPv6
client is started earlier than NDisc, by WithoutRA=solicit.

Fixes #31263.