dhcpv6-ia: apply prefix_filter on dhcpv6

dhcpv6-ia: apply prefix_filter on dhcpv6

The prefix_filter allows to select which prefix should be assigned
to clients if you have multiple prefixes on an interface.
Currently, the filter only applies to RAs and does work with
a dhcpv6 server.

This commit enables the filter also on dhcpv6.

Signed-off-by: Nick Hainke <vincent@systemli.org>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>