irqchip/bcm-6345-l1: 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.

The cpu variable is a pointer to "struct bcm6345_l1_cpu" and this structure
ends in a flexible array:

struct bcm6345_l1_cpu {
[...]
u32 enable_cache[];
};

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
kzalloc() function.

This way, the code is more readable and safer.

Signed-off-by: Erick Archer <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/162 [2]

drivers/irqchip/irq-bcm6345-l1.c

···
 	else if (intc->n_words != n_words)
 		return -EINVAL;
 
-	cpu = intc->cpus[idx] = kzalloc(sizeof(*cpu) + n_words * sizeof(u32),
+	cpu = intc->cpus[idx] = kzalloc(struct_size(cpu, enable_cache, n_words),
 					GFP_KERNEL);
 	if (!cpu)
 		return -ENOMEM;