ospf6d: correct nexthop through directly connected networks

ospf6d: correct nexthop through directly connected networks

This is implementing this part of RFC 2328:

This is the "first case", see below,

16.1.1. The next hop calculation

...
If there is at least one intervening router in the current
shortest path between the destination and the root, the
destination simply inherits the set of next hops from the
parent. Otherwise, there are two cases. In the first case,
the parent vertex is the root (the calculating router
itself). This means that the destination is either a
directly connected network or directly connected router.
The outgoing interface in this case is simply the OSPF
interface connecting to the destination network/router.
...

The current Quagga code always tries to inherit the nexthop from
a parent vertex, but does not cover the case that the destination
is directly connected to the root vertex. This patch adds support
for that case.

Signed-off-by: James Li <jli at cumulusnetworks.com>
Reviewed-by: Dinesh G Dutt <ddutt at cumulusnetworks.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>