overlay: Test lost immutable/append-only flags on copy-up

overlay: Test lost immutable/append-only flags on copy-up

Chengguang Xu reported [1] that append-only flag is lost on copy-up.
I had noticed that for directories, immutable flag can also be lost
on copy up (when parent is copied up). That's an old overlayfs bug.

Fixing this requires some VFS API changes that Miklos has proposed[2]

[1] https://lore.kernel.org/linux-unionfs/20201226104618.239739-1-cgxu519@mykernel.net/
[2] https: //lore.kernel.org/linux-fsdevel/20210203124112.1182614-1-mszeredi@redhat.com/

[Eryu: referencing the thread of the pending fix]

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>