md: raid10: Use struct_size() in kmalloc()

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[];
};

instance = kmalloc(size, GFP_KERNEL);

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

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

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/raid10.c

@@ -4755,8 +4755,7 @@ static int handle_reshape_read_error(struct mddev *mddev,
 	int idx = 0;
 	struct page **pages;
 
-	r10b = kmalloc(sizeof(*r10b) +
-	       sizeof(struct r10dev) * conf->copies, GFP_NOIO);
+	r10b = kmalloc(struct_size(r10b, devs, conf->copies), GFP_NOIO);
 	if (!r10b) {
 		set_bit(MD_RECOVERY_INTR, &mddev->recovery);
 		return -ENOMEM;