net_sched: Use struct_size() and flex_array_size() helpers

Make use of the struct_size() and flex_array_size() helpers instead of
an open-coded version, in order to avoid any potential type mistakes
or integer overflows that, in the worse scenario, could lead to heap
overflows.

Link: https://github.com/KSPP/linux/issues/160Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210928193107.GA262595@embeddedorSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_api.c

@@ -507,20 +507,21 @@ static struct qdisc_size_table *qdisc_get_stab(struct nlattr *opt,
 	list_for_each_entry(stab, &qdisc_stab_list, list) {
 		if (memcmp(&stab->szopts, s, sizeof(*s)))
 			continue;
-		if (tsize > 0 && memcmp(stab->data, tab, tsize * sizeof(u16)))
+		if (tsize > 0 &&
+		    memcmp(stab->data, tab, flex_array_size(stab, data, tsize)))
 			continue;
 		stab->refcnt++;
 		return stab;
 	}
 
-	stab = kmalloc(sizeof(*stab) + tsize * sizeof(u16), GFP_KERNEL);
+	stab = kmalloc(struct_size(stab, data, tsize), GFP_KERNEL);
 	if (!stab)
 		return ERR_PTR(-ENOMEM);
 
 	stab->refcnt = 1;
 	stab->szopts = *s;
 	if (tsize > 0)
-		memcpy(stab->data, tab, tsize * sizeof(u16));
+		memcpy(stab->data, tab, flex_array_size(stab, data, tsize));
 
 	list_add_tail(&stab->list, &qdisc_stab_list);