bonding: Don't allow mode change via sysfs with slaves present

When changing mode via bonding's sysfs, the slaves are not initialized
correctly. Forbid to change modes with slaves present to ensure that every
slave is initialized correctly via bond_enslave().

Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Acked-by: Nicolas de Pesloüan <nicolas.2p.debian@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>