net/ipv6: Do not allow route add with a device that is down

IPv6 allows routes to be installed when the device is not up (admin up).
Worse, it does not mark it as LINKDOWN. IPv4 does not allow it and really
there is no reason for IPv6 to allow it, so check the flags and deny if
device is admin down.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>