mm: kmem: reimplement get_obj_cgroup_from_current()

Reimplement get_obj_cgroup_from_current() using current_obj_cgroup(). 
get_obj_cgroup_from_current() and current_obj_cgroup() share 80% of the
code, so the new implementation is almost trivial.

get_obj_cgroup_from_current() is a convenient function used by the
bpf subsystem, so there is no reason to get rid of it completely.

Link: https://lkml.kernel.org/r/20231019225346.1822282-7-roman.gushchin@linux.dev


Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Shakeel Butt <shakeelb@google.com>
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>
Cc: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>