[17.0-stable] usbmanager: stop self-parent assigngrp from OOMing pillar

[eriknordmark]

Description

Backport of #6184 to 17.0-stable.

A device model can declare a PhysicalIO whose parentassigngrp equals its own
assigngrp. When usbmanager processes such a bundle it builds a
self-referential node in its internal ioBundle tree (the group becomes its own
child), and the dependents walk then recurses on that self-loop with no visited
guard. The goroutine stack grows without bound until the pillar memory cgroup
OOM-kills zedbox, and the watchdog reboots the device — repeatedly, since the
offending config is re-applied on every boot, leaving the device stuck in a
reboot loop and unmanageable.

The existing circular-dependency check in addIOBundle cannot catch a
self-parent, because a group is never among its own descendants before it is
inserted. This change rejects a self-parent bundle in addIOBundle, and guards
both tree walks (groupDependendentsImpl and groupParents) with a visited
set so any cyclic parentassigngrp graph that reaches them terminates.

Cherry-picked with git cherry-pick -x from
00a7df8768e80a0726996e1dc84fe1434a2cae62, the merge of #6184 on master. It
applies with no adaptation: pkg/pillar/cmd/usbmanager/ioBundleTree.go and its
test file have not been touched since the file was added in 31b9c8eab
(Dec 2023), so both are byte-identical to master on this branch and the
resulting diff matches #6184 exactly.

How to test and validate this PR

Covered by unit tests in pkg/pillar/cmd/usbmanager/ioBundleTree_test.go
(TestSelfParentAssigngrpRejected, TestTreeWalksGuardCycles).

Both were run against this branch's pkg/pillar before and after the fix.
Without it, TestSelfParentAssigngrpRejected reports that the self-parent
bundle was added, and TestTreeWalksGuardCycles does not terminate — the run
either trips the test's 2 s guard or dies with fatal error: stack overflow.
With the fix, the full ./cmd/usbmanager/ package passes, along with go vet
and gofmt.

End to end: push a device model with a PhysicalIO whose parentassigngrp
equals its assigngrp. On an unfixed build the device OOM-reboots within
~2 minutes and loops; with this fix the model is reported as an error
(IOBundle cannot be its own parent) and the device stays up with steady
memory.

Changelog notes

Fixed a device reboot loop that could occur when the device model contained an
I/O adapter whose parent assignment group referred to itself.

PR Backports

All four current LTS branches carry the affected code byte-identical to master,
so all of them are affected. This PR is one of a set of four:

• 17.0-stable: this PR.
• 16.0-stable: [16.0-stable] usbmanager: stop self-parent assigngrp from OOMing pillar #6254
• 14.5-stable: [14.5-stable] usbmanager: stop self-parent assigngrp from OOMing pillar #6255
• 13.4-stable: [13.4-stable] usbmanager: stop self-parent assigngrp from OOMing pillar #6256

Checklist

• I've provided a proper description
• I've added the proper documentation — not needed, no user-facing
  configuration surface changes
• I've tested my PR on amd64 device — not re-tested on this branch; the
  identical change was verified on amd64 in usbmanager: stop self-parent assigngrp from OOMing pillar #6184, and the unit tests above
  were run against this branch
• I've tested my PR on arm64 device
• I've written the test verification instructions
• I've set the proper labels to this PR

For backport PRs (remove it if it's not a backport):

• I've added a reference link to the original PR
• PR's title follows the template

And the last but not least:

• I've checked the boxes above, or I've provided a good reason why I didn't
  check them.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 21.03%. Comparing base (2de292d) to head (d9d0a80).
⚠️ Report is 3 commits behind head on 17.0-stable.

Additional details and impacted files

@@               Coverage Diff               @@
##           17.0-stable    #6253      +/-   ##
===============================================
+ Coverage        20.61%   21.03%   +0.42%     
===============================================
  Files              506      516      +10     
  Lines            92880    94590    +1710     
===============================================
+ Hits             19148    19900     +752     
- Misses           72136    72915     +779     
- Partials          1596     1775     +179     

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