[NET] back: Replace netif->status with netif_carrier_ok

[NET] back: Replace netif->status with netif_carrier_ok

The connection status to the frontend can be represented using
netif_carrier_ok instead of netif->status.  As a result, we delay
the construction of the dev qdisc until the carrier comes on.  This
is a prerequisite for adding a tx queue.

By the same token, netif->active is now simply the conjunction of
netif_running and netif_carrier_ok so it too can be removed.

Because netif_carrier_off/netif_carrier_on and rtnl_lock all entail
memory barriers, there is no need to have extra memory barriers around
them.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>