slab: make __slab_free() more clear

The function is tricky and many of its tests are hard to understand. Try
to improve that by using more descriptively named variables and added
comments.

- rename 'prior' to 'old_head' to match the head and tail parameters
- introduce a 'bool was_full' to make it more obvious what we are
  testing instead of the !prior and prior tests
- add or improve comments in various places to explain what we're doing

Also replace kmem_cache_has_cpu_partial() tests with
IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL) which are compile-time constants.
We can do that because the kmem_cache_debug(s) case is handled upfront
via free_to_partial_list().

Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20251105-sheaves-cleanups-v1-1-b8218e1ac7ef@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

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