mm: kmem: scoped objcg protection

Switch to a scope-based protection of the objcg pointer on slab/kmem
allocation paths.  Instead of using the get_() semantics in the
pre-allocation hook and put the reference afterwards, let's rely on the
fact that objcg is pinned by the scope.

It's possible because:
1) if the objcg is received from the current task struct, the task is
   keeping a reference to the objcg.
2) if the objcg is received from an active memcg (remote charging),
   the memcg is pinned by the scope and has a reference to the
   corresponding objcg.

Link: https://lkml.kernel.org/r/20231019225346.1822282-5-roman.gushchin@linux.dev


Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>