blk-iocost: Use struct_size() in kzalloc_node()

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

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Addresses-KSPP-ID: https://github.com/KSPP/linux/issues/83Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -2045,8 +2045,7 @@ static struct blkg_policy_data *ioc_pd_alloc(gfp_t gfp, struct request_queue *q,
 	int levels = blkcg->css.cgroup->level + 1;
 	struct ioc_gq *iocg;
 
-	iocg = kzalloc_node(sizeof(*iocg) + levels * sizeof(iocg->ancestors[0]),
-			    gfp, q->node);
+	iocg = kzalloc_node(struct_size(iocg, ancestors, levels), gfp, q->node);
 	if (!iocg)
 		return NULL;