selftests: cgroup: fix unexpected failure on test_memcg_sock

selftests: cgroup: fix unexpected failure on test_memcg_sock

Before server got a client connection, there were some memory allocations
in the test memcg, such as user stack.  So do not count those allocations
which are not related to socket when checking socket memory accounting.

Link: https://lkml.kernel.org/r/20230619124735.2124-1-haifeng.xu@shopee.com
Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>