Netlink: Ignore dead routes

With net.ipv4.conf.XXX.ignore_routes_with_linkdown sysctl, a user can
ensure the kernel does not use a route whose target interface is down.
Such route is marked with a 'dead' / RTNH_F_DEAD flag.

Ignore these routes or multipath nexthops during scan.

Thanks to Vincent Bernat for the original patch.