interface-ip: add unreachable route if address is offlink

In order to avoid a routing loop add an unreachable route for the
address prefix is the offlink flag is set for an address.
This fixes a routing loop which is currently present on point-to-point
links (e.g PPP) when the wan interface is assigned a globally unique
prefix (e.g. 2001:db8:1:0::/64) from which an IPv6 address is picked
and installed on the wan interface
(e.g. 2001:db8:1:0:5054:ff:feab:d87c/64)

The prefix route 2001:db8:1::/64 would be present in the routing table
which will route any packet with as destination 2001:db8:1::/64 to the wan
interface and would be routed back by the upstream router due to the
wan interface due to the assigned global unique prefix.
Besides not installing the prefix route 2001:db8:1::/64 on point-to-point links
adding an unreachable route is required to avoid the routing loop.

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

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