[IPV6]: Set IF_READY if the device is up and has carrier

We still need to set the IF_READY flag in ipv6_add_dev for the case
where all addresses (including the link-local) are deleted and then
recreated.  In that case the IPv6 device too will be destroyed and
then recreated.

In order to prevent the original problem, we simply ensure that
the device is up before setting IF_READY.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>