ASoC: apple: mca: Annotate struct mca_data 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 checking 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 mca_data.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Martin Povišer <povik+lin@cutebit.org>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: asahi@lists.linux.dev
Cc: alsa-devel@alsa-project.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922175050.work.819-kees@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/apple/mca.c

@@ -161,7 +161,7 @@ struct mca_data {
 	struct mutex port_mutex;
 
 	int nclusters;
-	struct mca_cluster clusters[];
+	struct mca_cluster clusters[] __counted_by(nclusters);
 };
 
 static void mca_modify(struct mca_cluster *cl, int regoffset, u32 mask, u32 val)