Check the F_NEXTHOP flag on the right kroute6 object.

Check the F_NEXTHOP flag on the right kroute6 object.

On multipath routes the check ended up checking the wrong route for the
nexthop update. This resulted in a use-after-free in kroute_detach_nexthop().
This only affects IPv6 in the IPv4 code path the right object was already used.

Thanks to sthen@ for providing the debug information to track this down.
OK sthen@ tb@