bonding: get rid of IFF_SLAVE_INACTIVE netdev->priv_flag

Since bond-related code was moved from net/core/dev.c into bonding,
IFF_SLAVE_INACTIVE is no longer needed. Replace is with flag "inactive"
stored in slave structure

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Reviewed-by: Nicolas de Pesloüan <nicolas.2p.debian@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>