isisd: apply fast-reroute when an interface falls down

isisd: apply fast-reroute when an interface falls down

Backup routes are sent to zebra by routing daemons such as isisd so that
the dataplane can pre-install them with a lower priority. When an
interface comes down, the associated primary routes are discarded by the
dataplane and the backup ones take over.

However, some dataplanes (e.g. Netlink ones) do not pre-install the
backup routes. Associated prefixes have no next-hop until SPF is
recomputed.

Apply fast-reroute as soon as an interface falls down by sending route
UPDATEs to zebra.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>