[IPV6]: Defer IPv6 device initialization until a valid qdisc is specified

To judge the timing for DAD, netif_carrier_ok() is used. However,
there is a possibility that dev->qdisc stays noop_qdisc even if
netif_carrier_ok() returns true. In that case, DAD NS is not sent out.
We need to defer the IPv6 device initialization until a valid qdisc
is specified.

Signed-off-by: Mitsuru Chinen <mitch@linux.vnet.ibm.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>