mtd: rawnand: Prefer struct_size over open coded arithmetic

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1].

As the "chip" variable is a pointer to "struct mtk_nfc_nand_chip" and
this structure ends in a flexible array:

struct mtk_nfc_nand_chip {
	[...]
	u8 sels[] __counted_by(nsels);
};

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the argument "size + count * size" in the
devm_kzalloc() function.

This way, the code is more readable and safer.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: KSPP#160 [2]
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Signed-off-by: Erick Archer <[email protected]>
Acked-by: Uwe Kleine-König <[email protected]>
Reviewed-by: AngeloGioacchino Del Regno <[email protected]>
Reviewed-by: Matthias Brugger <[email protected]>
Signed-off-by: Miquel Raynal <[email protected]>
Link: https://lore.kernel.org/linux-mtd/[email protected]

Author: Erick Archer

drivers/mtd/nand/raw/mtk_nand.c

@@ -1356,7 +1356,7 @@ static int mtk_nfc_nand_chip_init(struct device *dev, struct mtk_nfc *nfc,
 		return -EINVAL;
 	}
 
-	chip = devm_kzalloc(dev, sizeof(*chip) + nsels * sizeof(u8),
+	chip = devm_kzalloc(dev, struct_size(chip, sels, nsels),
 			    GFP_KERNEL);
 	if (!chip)
 		return -ENOMEM;