chroot createPlatformContainer: use MS_REMOUNT

[nalind]

What type of PR is this?

/kind bug

What this PR does / why we need it:

When setting mount propagation on the root mount before unmounting it, use MS_REBIND, since we know it's already a bind mount, and we actually want to affect the extant bind mount instead of creating another right over it. Otherwise, we might as well have not bothered.

How to verify it

New integration test!

Which issue(s) this PR fixes:

Related to KONFLUX-5871.

Special notes for your reviewer:

Does this PR introduce a user-facing change?

None

[flouthoc]

LGTM

[flouthoc]

@nalind This needs a rebase.

[nalind]

Rebased.

[flouthoc]

/lgtm
/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: flouthoc, nalind

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[nalind]

/cherry-pick release-1.39
@TomSweeneyRedHat that should fix #6001/RHEL-80446 and #5874 (comment).

[openshift-cherrypick-robot]

@nalind: #5992 failed to apply on top of branch "release-1.39":

Applying: chroot createPlatformContainer: use MS_REMOUNT
Using index info to reconstruct a base tree...
M	tests/run.bats
Falling back to patching base and 3-way merge...
Auto-merging tests/run.bats
CONFLICT (content): Merge conflict in tests/run.bats
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"
Patch failed at 0001 chroot createPlatformContainer: use MS_REMOUNT

In response to this:

/cherry-pick release-1.39
@TomSweeneyRedHat that should fix #6001/RHEL-80446 and #5874 (comment).

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