mm: percpu: fix incorrect size in pcpu_obj_full_size()

mm: percpu: fix incorrect size in pcpu_obj_full_size()

The extra space which is used to store the obj_cgroup membership is only
valid when kmemcg is enabled.  The kmemcg can be disabled via the kernel
parameter "cgroup.memory=nokmem" at boot time.  This helper is also used
in non-memcg code, for example the tracepoint, so we should fix it.

It was found by code review when I was implementing bpf memory usage[1].
No real issue happens in production environment.

[1]. https://lwn.net/Articles/921991/

Link: https://lkml.kernel.org/r/20230214153549.12291-1-laoar.shao@gmail.com
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Vasily Averin <vvs@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>