bonding: re-evaluate force_primary when the primary slave name changes

There is a timing issue under active-standy mode, when bond_enslave() is
called, bond->params.primary might not be initialized yet.

Any time the primary slave string changes, bond->force_primary should be
set to true to make sure the primary becomes the active slave.

Signed-off-by: Xiangning Yu <yuxiangning@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>