block/wbt: enable wbt after switching cfq to other schedulers

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6LH5K


CVE: NA

--------------------------------

Commit 80061216 ("block/wbt: fix negative inflight counter
when remove scsi device") move wbt_enable_default() from
elv_unregister_queue() to bfq_exit_queue(). As the result of
it, wbt can't be enabled when we switch cfq to other schedulers.

Fixes: 80061216 ("block/wbt: fix negative inflight counter when remove scsi device")

Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>