net: Update IP address refcount properly when address already exists

If an IP address already exists when it is tried to be added to the
network interface, then just return it but update ref count if it was
not updated. This could happen if the address was added and then removed,
but for example an active connection was still using it and keeping the
ref count > 0. In this case we must update the ref count so that the IP
address is not removed if the connection is closed.

Fixes #85380

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>