bonding: fail_over_mac should only affect AB mode in bond_set_mac_address()

The fail_over_mac could be set to active or follow in any time for all modes,
so if the fail_over_mac is not none and the current mode is not active-backup,
the bond_set_mac_address() could not change the master and slave's MAC address.

In bond_set_mac_address(), the fail_over_mac should only affect AB mode, so modify
to check the mode in addition to fail_over_mac when setting bond's MAC address.

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