Fixes rootfs undefined behaviour from issue #863

[nuclearcat]

Tested locally, now the rootfs.ext4 image is without errors.

[gctucker]

@nuclearcat Please reformat the Git commit to follow the convention. For example (adjust with your own words...):

config/rootfs/debos: fix ext4 corrupt images

Fix ext4 file system image corruption by adding the postprocess
attribute to the steps that move the image file.

Link: https://github.com/kernelci/kernelci-core/issues/863
Signed-off-by: Denys Fedoryshchenko <[email protected]>

[nuclearcat]

I think done. I hope i did not miss anything.

[gctucker]

Great. Have you tested this version locally as well, with the code blocks at the same place in the file?

We should have some rootfs images built on staging later today or tomorrow to confirm there's no unwanted side-effects in any case.

[nuclearcat]

Not yet, I will test it in the next few hours.

[nuclearcat]

All tests passed fine, image is bootable, dumpe2fs is clean.

[gctucker]

Great, thanks. I've started a buster amd64 build on staging, will check tomorrow if it worked as expected.

[gctucker]

Here's the staging build:
https://storage.staging.kernelci.org/images/rootfs/debian/buster/20211109.1/amd64/

Tested OK:
https://lava.collabora.co.uk/scheduler/job/4987676