netfilter: nf_ct_ipv6: use ipv6_iface_scope_id in conntrack to return scope id

As in (842df073 ipv6: use newly introduced __ipv6_addr_needs_scope_id and
ipv6_iface_scope_id).

Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>