bonding: remove unwanted lock for bond_option_active_slave_set()

The bond_option_active_slave_set() is always called in RTNL,
the RTNL could protect bond slave list, so remove the unwanted
bond lock.

Suggested-by: Jay Vosburgh <fubar@us.ibm.com>
Suggested-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>