selftests/bpf: Use bpf_core_enum_value for stats in cgroup_iter_memcg

Replace hardcoded enum values with bpf_core_enum_value() calls in
cgroup_iter_memcg test to improve portability across different
kernel versions.

The change adds runtime enum value resolution for:
- node_stat_item: NR_ANON_MAPPED, NR_SHMEM, NR_FILE_PAGES,
  NR_FILE_MAPPED
- vm_event_item: PGFAULT

This ensures the BPF program can adapt to enum value changes
between kernel versions.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Reviewed-by: JP Kobryn <jp.kobryn@linux.dev>
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
Link: https://lore.kernel.org/r/ca6eb1a1a4fd7a17ffe995acf52c9a4ceb7bac13.1772505399.git.zhuhui@kylinos.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: teawater

tools/testing/selftests/bpf/progs/cgroup_iter_memcg.c

@@ -26,11 +26,18 @@ int cgroup_memcg_query(struct bpf_iter__cgroup *ctx)
 
 	bpf_mem_cgroup_flush_stats(memcg);
 
-	memcg_query.nr_anon_mapped = bpf_mem_cgroup_page_state(memcg, NR_ANON_MAPPED);
-	memcg_query.nr_shmem = bpf_mem_cgroup_page_state(memcg, NR_SHMEM);
-	memcg_query.nr_file_pages = bpf_mem_cgroup_page_state(memcg, NR_FILE_PAGES);
-	memcg_query.nr_file_mapped = bpf_mem_cgroup_page_state(memcg, NR_FILE_MAPPED);
-	memcg_query.pgfault = bpf_mem_cgroup_vm_events(memcg, PGFAULT);
+	memcg_query.nr_anon_mapped = bpf_mem_cgroup_page_state(
+		memcg,
+		bpf_core_enum_value(enum node_stat_item, NR_ANON_MAPPED));
+	memcg_query.nr_shmem = bpf_mem_cgroup_page_state(
+		memcg, bpf_core_enum_value(enum node_stat_item, NR_SHMEM));
+	memcg_query.nr_file_pages = bpf_mem_cgroup_page_state(
+		memcg, bpf_core_enum_value(enum node_stat_item, NR_FILE_PAGES));
+	memcg_query.nr_file_mapped = bpf_mem_cgroup_page_state(
+		memcg,
+		bpf_core_enum_value(enum node_stat_item, NR_FILE_MAPPED));
+	memcg_query.pgfault = bpf_mem_cgroup_vm_events(
+		memcg, bpf_core_enum_value(enum vm_event_item, PGFAULT));
 
 	bpf_put_mem_cgroup(memcg);