hwspinlock: stm32: Use struct_size() helper in devm_kzalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/hwspinlock/stm32_hwspinlock.c:84:32: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Baolin Wang <[e-mail]>
Signed-off-by: Bjorn Andersson <[e-mail]>
Link: https://lore.kernel.org/r/20220125021353.GA29777@embeddedor

drivers/hwspinlock/stm32_hwspinlock.c

···
 	struct device *dev = &pdev->dev;
 	struct stm32_hwspinlock *hw;
 	void __iomem *io_base;
-	size_t array_size;
 	int i, ret;
 
 	io_base = devm_platform_ioremap_resource(pdev, 0);
 	if (IS_ERR(io_base))
 		return PTR_ERR(io_base);
 
-	array_size = STM32_MUTEX_NUM_LOCKS * sizeof(struct hwspinlock);
-	hw = devm_kzalloc(dev, sizeof(*hw) + array_size, GFP_KERNEL);
+	hw = devm_kzalloc(dev, struct_size(hw, bank.lock, STM32_MUTEX_NUM_LOCKS), GFP_KERNEL);
 	if (!hw)
 		return -ENOMEM;