selftests: cgroup: Add 'malloc' failures checks in test_memcontrol

There are several 'malloc' calls in test_memcontrol, which can be
unsuccessful. This patch will add 'malloc' failures checking to
give more details about test's fail reasons and avoid possible
undefined behavior during the future null dereference (like the
one in alloc_anon_50M_check_swap function).

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>