ra: fix routing loop on point to point links

ra: fix routing loop on point to point links

For point-to-point links (e.g. PPP) don't create a link prefix route
when receiving a prefix information option with the on-link flag set.
Point-to-point links are non shared media and as such a destination
IPv6 address cannot be on-link.
If a link prefix route points to a point-to-point link it can trigger
a routing loop if the destination IPv6 address belongs to the prefix.
If such a packet is received and not directed to a local IPv6 address
it will be routed to the point-to-point link due to the link prefix route;
the upstream ISP router will route the IPv6 packet back due to the assigned
prefix route creating a "ping pong" effect

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>