block/blk-iocost: ensure 'ret' is set on error

In case blkg_conf_open_bdev_frozen() fails, ioc_qos_write() jumps to the
error path without assigning a value to 'ret'. Ensure that it inherits
the error from the passed back error value.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202503200454.QWpwKeJu-lkp@intel.com/
Fixes: 9730763 ("block: correct locking order for protecting blk-wbt parameters")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -3229,8 +3229,10 @@ static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
 	blkg_conf_init(&ctx, input);
 
 	memflags = blkg_conf_open_bdev_frozen(&ctx);
-	if (IS_ERR_VALUE(memflags))
+	if (IS_ERR_VALUE(memflags)) {
+		ret = memflags;
 		goto err;
+	}
 
 	body = ctx.body;
 	disk = ctx.bdev->bd_disk;