fc: use restrictive file permissions for sysfs writes

[majiayu000]

Summary

• Changed file permission mode from 0666 (world-writable) to 0200 (write-only for owner) when writing to sysfs pseudo-files in the FC volume plugin
• Added comments explaining the rationale for the permission mode choice

Details

The FC volume plugin uses WriteFile to write to sysfs pseudo-files at:

• /sys/block/<device>/device/delete - to remove SCSI devices
• /sys/class/scsi_host/<host>/scan - to rescan SCSI bus

While sysfs files have kernel-defined permissions that override the specified mode, using 0200 better reflects the intent that only root should write to these files. This addresses the misleading use of world-writable permissions (0666) in the codebase.

This is a partial fix for the security audit finding in issue #81116 (TOB-K8S-004: Pervasive world-accessible file permissions).

Fixes #81116

Test plan

• Ran go test ./pkg/volume/fc/... - all tests pass
• The change is cosmetic (sysfs files already have kernel-defined permissions) so no functional impact is expected

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: majiayu000
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