ospf6d: fix setting of the forwarding address in AS-External LSAs

ospf6d: fix setting of the forwarding address in AS-External LSAs

When redistributing a local route, ensure the forwarding address field
of AS-External LSAs is only set when the associated nexthop interface
is OSPF-enabled. This is necessary because the OSPF RFC requires that
the AS-External LSA forwarding address should be resolvable using an
intra-area or inter-area path.

Fixes #14462.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>