bonding: bond_change_active_slave() cleanup under active-backup

simplified the code of bond_change_active_slave() such that under
active-backup mode there's one "if (new_active)" test and the rest
of the code only does extra checks on top of it. This removed an
unneeded "if (bond->send_grat_arp > 0)" check and avoid calling
bond_send_gratuitous_arp when there's no active slave.

Jay Vosburgh made minor coding style changes to the orignal patch.

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>