bonding: remove NULL verification from bond_get_bond_by_slave()

Every caller relies on the result being the actual bond, so this
verification just masks the real problem.

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>