mm: define obj_cgroup_get() if CONFIG_MEMCG is not defined

mm: define obj_cgroup_get() if CONFIG_MEMCG is not defined

This resolves an issue with obj_cgroup_get() not being defined if
CONFIG_MEMCG is not defined.

Before this patch, we would see build errors if obj_cgroup_get() is called
from code that is agnostic of CONFIG_MEMCG.

The zswap_store() changes for large folios in subsequent commits will
require the use of obj_cgroup_get() in zswap code that falls into this
category.

Link: https://lkml.kernel.org/r/20241001053222.6944-2-kanchana.p.sridhar@intel.com
Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Wajdi Feghali <wajdi.k.feghali@intel.com>
Cc: "Zou, Nanhai" <nanhai.zou@intel.com>
Cc: Barry Song <21cnbao@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>