writeback: remove redundant checks for root memcg

The check for root memcg will be done in wb_get_lookup(), so remove the
redundant one to simplify the code.

Link: https://lkml.kernel.org/r/20230808084431.1632934-1-alexjlzheng@tencent.com
Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/backing-dev.c

@@ -732,9 +732,6 @@ struct bdi_writeback *wb_get_create(struct backing_dev_info *bdi,
 
 	might_alloc(gfp);
 
-	if (!memcg_css->parent)
-		return &bdi->wb;
-
 	do {
 		wb = wb_get_lookup(bdi, memcg_css);
 	} while (!wb && !cgwb_create(bdi, memcg_css, gfp));