bpf: Annotate struct bpf_stack_map with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle [1], add __counted_by for struct bpf_stack_map.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci [1]
Link: https://lore.kernel.org/bpf/20231006201657.work.531-kees@kernel.org

kernel/bpf/stackmap.c

@@ -28,7 +28,7 @@ struct bpf_stack_map {
 	void *elems;
 	struct pcpu_freelist freelist;
 	u32 n_buckets;
-	struct stack_map_bucket *buckets[];
+	struct stack_map_bucket *buckets[] __counted_by(n_buckets);
 };
 
 static inline bool stack_map_use_build_id(struct bpf_map *map)