sched: act_ct: avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Remove unnecessary flex-array member `pad[]` and refactor the related
code a bit.

Fix the following warning:
net/sched/act_ct.c:57:29: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/ZrY0JMVsImbDbx6r@cuteSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/act_ct.c

@@ -44,8 +44,6 @@ static DEFINE_MUTEX(zones_mutex);
 struct zones_ht_key {
 	struct net *net;
 	u16 zone;
-	/* Note : pad[] must be the last field. */
-	u8  pad[];
 };
 
 struct tcf_ct_flow_table {
@@ -62,7 +60,7 @@ struct tcf_ct_flow_table {
 static const struct rhashtable_params zones_params = {
 	.head_offset = offsetof(struct tcf_ct_flow_table, node),
 	.key_offset = offsetof(struct tcf_ct_flow_table, key),
-	.key_len = offsetof(struct zones_ht_key, pad),
+	.key_len = offsetofend(struct zones_ht_key, zone),
 	.automatic_shrinking = true,
 };