dmaengine: apple-admac: Annotate struct admac_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 admac_data.

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

Cc: Vinod Koul <vkoul@kernel.org>
Cc: Hector Martin <marcan@marcan.st>
Cc: Sven Peter <sven@svenpeter.dev>
Cc: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Cc: asahi@lists.linux.dev
Cc: linux-arm-kernel@lists.infradead.org
Cc: dmaengine@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230817235859.49846-1-keescook@chromium.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/apple-admac.c

@@ -128,7 +128,7 @@ struct admac_data {
 	int irq;
 	int irq_index;
 	int nchannels;
-	struct admac_chan channels[];
+	struct admac_chan channels[] __counted_by(nchannels);
 };
 
 struct admac_tx {