bpf: fix end-of-list detection in cgroup_storage_get_next_key()

list_next_entry() never returns NULL -- when the current element is the
last entry it wraps to the list head via container_of(). The subsequent
NULL check is therefore dead code and get_next_key() never returns
-ENOENT for the last element, instead reading storage->key from a bogus
pointer that aliases internal map fields and copying the result to
userspace.

Replace it with list_entry_is_head() so the function correctly returns
-ENOENT when there are no more entries.

Fixes: de9cbbaadb ("bpf: introduce cgroup storage maps")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Reviewed-by: Sun Jian <sun.jian.kdev@gmail.com>
Acked-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/20260403132951.43533-2-bestswngs@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/local_storage.c

@@ -270,7 +270,7 @@ static int cgroup_storage_get_next_key(struct bpf_map *_map, void *key,
 			goto enoent;
 
 		storage = list_next_entry(storage, list_map);
-		if (!storage)
+		if (list_entry_is_head(storage, &map->list, list_map))
 			goto enoent;
 	} else {
 		storage = list_first_entry(&map->list,