Merge branch 'slab/for-6.3/cleanups' into slab/for-linus

A bunch of cleanups for SLAB and SLUB:

- Use the standard idiom to get head page of folio (by SeongJae Park)
- Simplify and inline is_debug_pagealloc_cache() in SLAB (by lvqian)
- Remove unused variable in SLAB (by Gou Hao)