Minor fixes

[WaberZhuang]

What this PR does / why we need it:

• skip erofs fs meta when rootfs is writable
• fix & enhance device probe
• device waiting timeout 0.5s -> 5s
• drop log of Stat and Usage
• fix the unlabelled initial layer so that, after switching from OverlayFS to OverlayBD, the unlabelled preparation can be dealt with correctly.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[Copilot]

🟡 Changes recommended

The readiness jump loses the discovered device path, causing successful attachments to time out.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Improves writable-rootfs handling and device attachment reliability while reducing noisy logs.

Changes:

• Extends device polling and adds device-node readiness checks.
• Refines writable-mode selection and EROFS fallback.
• Removes routine Stat and Usage logs.

File summaries

File	Description
pkg/snapshot/storage.go	Extends and enhances device readiness probing.
pkg/snapshot/overlay.go	Updates writable-mode, EROFS, and logging behavior.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Balanced

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