pkg/util/syncutil: potential performance regression

pkg/util/syncutil [performance regression](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Nightlies_RoachprodMicrobenchmarkWeeklyWip/21080195?buildTab=log) on master @ [fdb92776bd279bee489ced860ce37f6c003827aa](https://github.com/cockroachdb/cockroach/commits/fdb92776bd279bee489ced860ce37f6c003827aa):

Performance regressions detected in package pkg/util

Comparison: v25.4.3 (71d85362) -> refs/heads/master (fdb92776)

Found 17 benchmark(s) with regressions ≥20%:

- [ ] [pkg/util/syncutil→LoadMostlyHits/*syncutil.DeepCopyMap-32 (sec/op): +47.20% (47.2%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=LoadMostlyHits/*syncutil.DeepCopyMap-32&package=pkg/util/syncutil)
- [ ] [pkg/util/span→BTreeIterSeekGE/count=65536-32 (sec/op): +32.00% (32.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=BTreeIterSeekGE/count=65536-32&package=pkg/util/span)
- [ ] [pkg/util/log→ExpensiveLogEnabled-32 (sec/op): +26.68% (26.7%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ExpensiveLogEnabled-32&package=pkg/util/log)
- [ ] [pkg/util/tracing→SpanCreation/detached-child=false-32 (sec/op): +24.86% (24.9%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=SpanCreation/detached-child=false-32&package=pkg/util/tracing)
- [ ] [pkg/util/syncutil→AdversarialAlloc/*syncutil.RWMutexMap-32 (sec/op): +23.61% (23.6%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=AdversarialAlloc/*syncutil.RWMutexMap-32&package=pkg/util/syncutil)
- [ ] [pkg/util/quotapool→ConcurrentIntPool/workers=512,quota=513-32 (sec/op): +22.84% (22.8%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ConcurrentIntPool/workers=512,quota=513-32&package=pkg/util/quotapool)
- [ ] [pkg/util/quotapool→ConcurrentIntPool/workers=512,quota=511-32 (sec/op): +22.64% (22.6%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ConcurrentIntPool/workers=512,quota=511-32&package=pkg/util/quotapool)
- [ ] [pkg/util/encoding→PeekType-32 (sec/op): +20.05% (20.1%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=PeekType-32&package=pkg/util/encoding)
- [ ] [pkg/util→FastIntMap/10x10-4/map-sized-32 (allocs/op): +200.00% (200.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/10x10-4/map-sized-32&package=pkg/util)
- [ ] [pkg/util→FastIntMap/32x15-10/map-sized-32 (allocs/op): +200.00% (200.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/32x15-10/map-sized-32&package=pkg/util)
- [ ] [pkg/util→FastIntMap/1000x1000-500/map-sized-32 (allocs/op): +150.00% (150.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/1000x1000-500/map-sized-32&package=pkg/util)
- [ ] [pkg/util→FastIntMap/100x100-50/map-sized-32 (allocs/op): +50.00% (50.0%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/100x100-50/map-sized-32&package=pkg/util)
- [ ] [pkg/util→FastIntMap/100x100-50/fastintmap-32 (allocs/op): +33.33% (33.3%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=FastIntMap/100x100-50/fastintmap-32&package=pkg/util)
- [ ] [pkg/util/parquet→ParquetWriter/int-32 (B/op): +45.10% (45.1%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ParquetWriter/int-32&package=pkg/util/parquet)
- [ ] [pkg/util/parquet→ParquetWriter/pg_lsn-32 (B/op): +44.21% (44.2%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ParquetWriter/pg_lsn-32&package=pkg/util/parquet)
- [ ] [pkg/util/parquet→ParquetWriter/time-32 (B/op): +43.62% (43.6%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ParquetWriter/time-32&package=pkg/util/parquet)
- [ ] [pkg/util/parquet→ParquetWriter/geography-32 (B/op): +20.38% (20.4%)](https://microbench.testeng.crdb.io/dashboard/?benchmark=ParquetWriter/geography-32&package=pkg/util/parquet)
Help

See also: [How To Investigate a Go Test Failure \(internal\)](https://cockroachlabs.atlassian.net/l/c/HgfXfJgM)

/cc @cockroachdb/unowned

Jira issue: CRDB-60118