mm/swapfile.c: use struct_size() in kvzalloc()

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 foo {
    int stuff;
    struct boo entry[];
};

size = sizeof(struct foo) + count * sizeof(struct boo); instance = kvzalloc(size, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can now use the new struct_size() helper:

instance = kvzalloc(struct_size(instance, entry, count), GFP_KERNEL);

Notice that, in this case, variable size is not necessary, hence it is removed.

This code was detected with the help of Coccinelle.

---
 mm/swapfile.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index cca8420b12db..fd9b0025ad00 100644
@@ -2713,9 +2713,8 @@ static struct swap_info_struct *alloc_swap_info(void)
 	struct swap_info_struct *p;
 	unsigned int type;
 	int i;
-	unsigned int size = sizeof(*p) + nr_node_ids * sizeof(struct plist_node);
 
-	p = kvzalloc(size, GFP_KERNEL);
+	p = kvzalloc(struct_size(p, avail_lists, nr_node_ids), GFP_KERNEL);
 	if (!p)
 		return ERR_PTR(-ENOMEM);
 

2.20.1