mm/backing-dev.c: fix an error handling path in 'cgwb_create()'

If the 'kmalloc' fails, we must go through the existing error handling
path.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Fixes: 52ebea749aae ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks")
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

mm/backing-dev.c

@@ -569,8 +569,10 @@ static int cgwb_create(struct backing_dev_info *bdi,
 
 	/* need to create a new one */
 	wb = kmalloc(sizeof(*wb), gfp);
-	if (!wb)
-		return -ENOMEM;
+	if (!wb) {
+		ret = -ENOMEM;
+		goto out_put;
+	}
 
 	ret = wb_init(wb, bdi, blkcg_css->id, gfp);
 	if (ret)