Add the 'k' prefix to id 21000. And id `u1000` in the third
idmapping should be mapped to `k31000`, not `u31000`.

Signed-off-by: Aiden Ma <jiaheng.ma@foxmail.com>
Link: https://lore.kernel.org/r/tencent_4E7B1F143E8051530C21FCADF4E014DCBB06@qq.com


Signed-off-by: Christian Brauner <brauner@kernel.org>