percpu: Use struct_size() helper

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct pcpu_alloc_info {
	...
        struct pcpu_group_info  groups[];
};

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following form:

sizeof(*ai) + nr_groups * sizeof(ai->groups[0])

with:

struct_size(ai, groups, nr_groups)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

mm/percpu.c

@@ -2125,7 +2125,7 @@ struct pcpu_alloc_info * __init pcpu_alloc_alloc_info(int nr_groups,
 	void *ptr;
 	int unit;
 
-	base_size = ALIGN(sizeof(*ai) + nr_groups * sizeof(ai->groups[0]),
+	base_size = ALIGN(struct_size(ai, groups, nr_groups),
 			  __alignof__(ai->groups[0].cpu_map[0]));
 	ai_size = base_size + nr_units * sizeof(ai->groups[0].cpu_map[0]);