Fix installation of IPv6 host route to VPN server when using iservice.

Fix installation of IPv6 host route to VPN server when using iservice.

The "prepare IPv6 route message to interactive service" was properly
handing the correct interface index (r->adapter_index) for this case,
but then always overwrote the gateway address with our magic tun/tap
fe80::8 value.  Only do this for "on tap adapter" routes.

Pinpointed by Selva Nair.

Trac #850

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20170319191049.23970-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14282.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 27740b376c1ca89a43dcff5c8309f1e1afecc5c9)