mtd: rawnand: rockchip: Use struct_size()

Use struct_size() instead of hand writing it.
This is less verbose and more robust.

While at it, 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).

Also remove a useless comment about the position of a flex-array in a
structure.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/481721c2c7fe570b4027dbe231d523961c953d5a.1696146232.git.christophe.jaillet@wanadoo.fr

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

@@ -158,8 +158,7 @@ struct rk_nfc_nand_chip {
 	u32 timing;
 
 	u8 nsels;
-	u8 sels[];
-	/* Nothing after this field. */
+	u8 sels[] __counted_by(nsels);
 };
 
 struct rk_nfc {
@@ -1119,7 +1118,7 @@ static int rk_nfc_nand_chip_init(struct device *dev, struct rk_nfc *nfc,
 		return -EINVAL;
 	}
 
-	rknand = devm_kzalloc(dev, sizeof(*rknand) + nsels * sizeof(u8),
+	rknand = devm_kzalloc(dev, struct_size(rknand, sels, nsels),
 			      GFP_KERNEL);
 	if (!rknand)
 		return -ENOMEM;