mtd: rawnand: atmel: Annotate struct atmel_nand 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 atmel_nand.

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

Cc: Tudor Ambarus <tudor.ambarus@linaro.org>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Cc: linux-mtd@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230915201219.never.352-kees@kernel.org

drivers/mtd/nand/raw/atmel/nand-controller.c

@@ -165,7 +165,7 @@ struct atmel_nand {
 	struct atmel_pmecc_user *pmecc;
 	struct gpio_desc *cdgpio;
 	int numcs;
-	struct atmel_nand_cs cs[];
+	struct atmel_nand_cs cs[] __counted_by(numcs);
 };
 
 static inline struct atmel_nand *to_atmel_nand(struct nand_chip *chip)