network: dhcp6-pd: exclude all explicitly specified subnet IDs when searching free IDs

When the upstream link gained a lease, then several downstream links may
not appear yet. Previously, the explicitly specified subnet ID for a
downstream link which appears later may be already assigned to an
interface which does not request specific subnet ID.

To avoid such situation, this makes all specified IDs are excluded when
searching free IDs.

As a side effect, we can avoid the second call of
dhcp6_pd_distribute_prefix().