[NET]: Hide the queue_mapping field inside netif_subqueue_stopped

Many places get the queue_mapping field from skb to pass it to the
netif_subqueue_stopped() which will be 0 in any case.

Make the helper that works with sk_buff

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>