bonding: use kobject_put instead of _del after kobject_add

Otherwise the name of the kobject isn't getting freed and other stuff from
kobject_cleanup() isn't getting called. kobject_put() will call
kobject_del() on its own in kobject_cleanup().

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>