selftests: cgroup: fix test_kmem_memcg_deletion kernel mem check

Currently, not all kernel memory usage is being accounted for. This
commit switches to using the kernel entry within memory.stat which
already includes kernel_stack, pagetables, and slab. The kernel entry
also includes vmalloc and other additional kernel memory use cases which
were missing.

Link: https://lkml.kernel.org/r/bvrhe2tpsts2azaroq4ubp2slawmop6orndsswrewuscw3ugvk@kmemmrttsnc7


Signed-off-by: Lucas Karpinski <lkarpins@redhat.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>