mtd: rawnand: Annotate struct mtk_nfc_nand_chip 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
mtk_nfc_nand_chip.

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Roger Quadros <rogerq@kernel.org>
Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Cai Huoqing <cai.huoqing@linux.dev>
Cc: Chuanhong Guo <gch981213@gmail.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Li Zetao <lizetao1@huawei.com>
Cc: linux-mtd@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Cc: linux-hardening@vger.kernel.org
Link: https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci [1]
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20231006201728.work.750-kees@kernel.org

drivers/mtd/nand/raw/mtk_nand.c

@@ -130,7 +130,7 @@ struct mtk_nfc_nand_chip {
 	u32 spare_per_sector;
 
 	int nsels;
-	u8 sels[];
+	u8 sels[] __counted_by(nsels);
 	/* nothing after this field */
 };