perf(mempool): Replace `sync.Mutex` with `sync.Once`

[chaudum]

What this PR does / why we need it:

sync.Once uses an atomic integer as gate instead of a Mutex.

goos: linux
goarch: amd64
pkg: github.com/grafana/loki/v3/pkg/util/mempool
cpu: 11th Gen Intel(R) Core(TM) i7-1185G7 @ 3.00GHz
             │  bench.old  │             bench.new              │
             │   sec/op    │   sec/op     vs base               │
Slab/1KB-8     138.5n ± 2%   125.1n ± 2%  -9.71% (p=0.000 n=10)
Slab/1MB-8     140.5n ± 3%   128.1n ± 2%  -8.83% (p=0.000 n=10)
Slab/128MB-8   143.0n ± 3%   130.4n ± 3%  -8.81% (p=0.000 n=10)
geomean        140.7n        127.8n       -9.12%

Checklist

• [x] Reviewed the CONTRIBUTING.md guide (required)
• [ ] Documentation added
• [x] Tests updated
• [x] Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• [ ] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [ ] For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• [ ] If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR