fix: ensure passive.img is sparse after install (backport #960)

[mergify]

Problem:
If passive.img is not a sparse file, it can result in too much disk space being used on the COS_STATE partition which causes problems for upgrades (see related issues).

Solution:
After installation, run fallocate --dig-holes to make passive.img sparse if it's not already.

Related Issue:
harvester/harvester#7457
harvester/harvester#7518

Test plan:

• Install Harvester
• Post-install, check disk usage of active.img and passive.img. They should be the same (about 1.7G):

  # du -sh /run/initramfs/cos-state/cOS/*
  1.7G	/run/initramfs/cos-state/cOS/active.img
  1.7G	/run/initramfs/cos-state/cOS/passive.img
  

• The fact that passive.img shrunk from 3.1G to 1.7G should be recorded in the install log, e.g.:

  # grep passive /oem/install/console.log | tail -n 3
  time="2025-02-11T11:59:38Z" level=info msg="[stdout]: Ensuring passive.img is sparse..."
  time="2025-02-11T11:59:38Z" level=info msg="[stdout]:   was: 3.1G\t/tmp/mnt/STATE/cOS/passive.img"
  time="2025-02-11T11:59:38Z" level=info msg="[stdout]:   now: 1.7G\t/tmp/mnt/STATE/cOS/passive.img"
  ```<hr>This is an automatic backport of pull request #960 done by [Mergify](https://mergify.com).
  

[w13915984028]

Thanks for this interesting fix, just a question:

To be more secure, should we test if ${STATEDIR}/cOS/passive.img) is there first? There is no passive.img definition on Harvester code, elemental has following information

https://github.com/rancher/elemental-toolkit/blob/50f6b4ec667767ef4e7c1036866a2d0180221044/pkg/constants/constants.go#L195

https://github.com/rancher/elemental-toolkit/blob/50f6b4ec667767ef4e7c1036866a2d0180221044/docs/content/en/docs/Getting%20started/upgrading.md?plain=1#L68

[tserong]

I'm not overly concerned about that here -- we run elemental install a few lines above sparsify_passive_img, and elemental install is what creates passive.img in the first place, so it will always be present at this point.