bonding: use bond_set_slave_arr to simplify code

In bond_reset_slave_arr(), values are assigned and memory is released only
when the variables "usable" and "all" are not NULL. But even if the
"usable" and "all" variables are NULL, they can still work, because value
will be checked in kfree_rcu. Therefore, use bond_set_slave_arr() and set
the input parameters "usable_slaves" and "all_slaves" to NULL to simplify
the code in bond_reset_slave_arr(). And the same to bond_uninit().

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>