dm raid1: Use struct_size() in kzalloc()

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 mirror_set {
	...
        struct mirror mirror[0];
};

size = sizeof(struct mirror_set) + count * sizeof(struct mirror); instance = kzalloc(size, GFP_KERNEL)

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

instance = kzalloc(struct_size(instance, mirror, count), GFP_KERNEL)

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

This code was detected with the help of Coccinelle.

---
 drivers/md/dm-raid1.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

index 5a51151f680d..f7226ab57fc8 100644
@@ -878,12 +878,9 @@ static struct mirror_set *alloc_context(unsigned int nr_mirrors,
 					struct dm_target *ti,
 					struct dm_dirty_log *dl)
 {
-	size_t len;
 	struct mirror_set *ms = NULL;
 
-	len = sizeof(*ms) + (sizeof(ms->mirror[0]) * nr_mirrors);
-
-	ms = kzalloc(len, GFP_KERNEL);
+	ms = kzalloc(struct_size(ms, mirror, nr_mirrors), GFP_KERNEL);
 	if (!ms) {
 		ti->error = "Cannot allocate mirror context";
 		return NULL;

2.22.0