dm thin metadata: Avoid returning cmd->bm wild pointer on error

Maybe __create_persistent_data_objects() caller will use PTR_ERR as a
pointer, it will lead to some strange things.
Signed-off-by: Ye Bin <yebin10@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-thin-metadata.c

@@ -739,12 +739,16 @@ static int __create_persistent_data_objects(struct dm_pool_metadata *pmd, bool f
 					  THIN_MAX_CONCURRENT_LOCKS);
 	if (IS_ERR(pmd->bm)) {
 		DMERR("could not create block manager");
-		return PTR_ERR(pmd->bm);
+		r = PTR_ERR(pmd->bm);
+		pmd->bm = NULL;
+		return r;
 	}
 
 	r = __open_or_format_metadata(pmd, format_device);
-	if (r)
+	if (r) {
 		dm_block_manager_destroy(pmd->bm);
+		pmd->bm = NULL;
+	}
 
 	return r;
 }