bonding: deliver netdev event for fail-over under the active-backup mode

under active-backup mode and when there's actual new_active slave,
have bond_change_active_slave() call the networking core to deliver
NETDEV_BONDING_FAILOVER event such that the fail-over can be notable
by code outside of the bonding driver such as the RDMA stack and
monitoring tools.

As the correct context of locking appropriate for notifier calls is RTNL
and nothing else, bond->curr_slave_lock and bond->lock are unlocked and
later locked again. This is ensured by the rest of the code to be safe
under backup-mode AND when new_active is not NULL.

Jay Vosburgh modified the original patch for formatting and fixed a
compiler error.

Signed-off-by: Or Gerlitz <ogerlitz@voltaire.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>