Skip the reference pool mutex on attach when no decrefs are pending

[tobni]

Closes #6199.

[codspeed-hq]

Merging this PR will degrade performance by 26.84%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 139 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	into_u128_max	1.5 µs	2.1 µs	-26.84%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing tobni:pool-dirty-flag (bf64a85) with main (8fcf8fc)}