dm: switch to rq-based after queue is initialized

hulk inclusion
category: bugfix
bugzilla: 187345, https://gitee.com/openeuler/kernel/issues/I5L5ZG
CVE: NA

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

Otherwise, null pointer crash can be triggered to handle bio in
blk_mq_submit_bio() while queue is not initialized.
Signed-off-by: NYu Kuai <yukuai3@huawei.com>
Reviewed-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

drivers/md/dm.c

@@ -2149,12 +2149,16 @@ int dm_setup_md_queue(struct mapped_device *md, struct dm_table *t)
 
 	switch (type) {
 	case DM_TYPE_REQUEST_BASED:
-		md->disk->fops = &dm_rq_blk_dops;
 		r = dm_mq_init_request_queue(md, t);
 		if (r) {
 			DMERR("Cannot initialize queue for request-based dm mapped device");
 			return r;
 		}
+		/*
+		 * Change the fops after queue is initialized, so that bio won't
+		 * issued by rq-based path until that.
+		 */
+		md->disk->fops = &dm_rq_blk_dops;
 		break;
 	case DM_TYPE_BIO_BASED:
 	case DM_TYPE_DAX_BIO_BASED: