Add priv_flag to specifically identify bonding-involved devices.  Needed
because IFF_MASTER is an unreliable identifier (vlan interfaces above bonding
will inherit IFF_MASTER).  Misidentification of devices would cause
notifier events for other devices to be erroneously processed by bonding,
causing various havoc.

Bug discovered by Martin Papik <martin.papik@ipsec.info>; this patch is
modified from his original.
Signed-off-by: Martin Papik <martin.papik@ipsec.info>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>