fix: correct cgroup v2 swap memory calculation (#3823)

[Elvand-Lie]

Fixes #3823

Problem:
In cgroup v2 environments, cAdvisor is currently reporting zero swap metrics even when swap is active. This is due to handler.go applying legacy cgroup v1 math (SwapUsage - Usage). In v2, memory.swap.current is decoupled from RAM usage, so subtracting Usage incorrectly zeroes out the metric.

Solution:
Updated the IsCgroup2UnifiedMode() block to assign SwapUsage.Usage directly to ret.Memory.Swap without the legacy subtraction.

Notes:

• Syntax verified with go fmt.
• Shoutout to @AkshatDudeja77 for the initial investigation and identifying the regression point in handler.go.

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[iholder101]

Thank you very much for addressing this!