Fix nexthop lookup for connected routes.

Fix nexthop lookup for connected routes.

In knexthop_true_nexthop() connected routes need to be handled specially.
If a nexthop is directly reachable the gateway needs to remain the same.
There is no nexthop set for F_CONNECTED kroutes. A very similar trick
is used in rde_rib.c::nexthop_update() but was forgotten here.

Regression noticed by Daniel Jakots (danj at chown.me).
OK tb@