net/bonding: Enable bonding to enslave netdevices not supporting set_mac_address()

This patch allows for enslaving netdevices which do not support
the set_mac_address() function. In that case the bond mac address is the one
of the active slave, where remote peers are notified on the mac address
(neighbour) change by Gratuitous ARP sent by bonding when fail-over occurs
(this is already done by the bonding code).

Signed-off-by: Moni Shoua <monis at voltaire.com>
Signed-off-by: Or Gerlitz <ogerlitz at voltaire.com>
Acked-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>