ipv6: Check nexthop flags in route dump instead of carrier

Similar to previous patch, there is no need to check for the carrier of
the nexthop device when dumping the route and we can instead check for
the presence of the RTNH_F_LINKDOWN flag.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>