ublk: Switch to memdup_user_nul() helper

Use memdup_user_nul() helper instead of open-coding
to simplify the code.
Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230815114815.1551171-1-ruanjinjie@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -2742,14 +2742,9 @@ static int ublk_ctrl_uring_cmd_permission(struct ublk_device *ub,
 	if (header->len < header->dev_path_len)
 		return -EINVAL;
 
-	dev_path = kmalloc(header->dev_path_len + 1, GFP_KERNEL);
-	if (!dev_path)
-		return -ENOMEM;
-
-	ret = -EFAULT;
-	if (copy_from_user(dev_path, argp, header->dev_path_len))
-		goto exit;
-	dev_path[header->dev_path_len] = 0;
+	dev_path = memdup_user_nul(argp, header->dev_path_len);
+	if (IS_ERR(dev_path))
+		return PTR_ERR(dev_path);
 
 	ret = -EINVAL;
 	switch (_IOC_NR(cmd->cmd_op)) {