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: NLi Lingfeng <lilingfeng3@huawei.com>
Reviewed-by: NYu Kuai <yukuai3@huawei.com>
Reviewed-by: NHou Tao <houtao1@huawei.com>
Signed-off-by: NZhang Changzhong <zhangchangzhong@huawei.com>

block/cfq-iosched.c

@@ -4563,6 +4563,7 @@ static void cfq_exit_queue(struct elevator_queue *e)
 	kfree(cfqd->root_group);
 #endif
 	kfree(cfqd);
+	wbt_enable_default(q);
 }
 
 static int cfq_init_queue(struct request_queue *q, struct elevator_type *e)