netfilter: x_tables: Avoid a couple -Wflex-array-member-not-at-end warnings

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

Use the TRAILING_OVERLAP() helper to fix the following warnings:

1 net/netfilter/x_tables.c:816:39: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] 1 net/netfilter/x_tables.c:811:39: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

This helper creates a union between a flexible-array member (FAM) and a set of members that would otherwise follow it. This overlays the trailing members onto the FAM while preserving the original memory layout.

---
Changes in v3:
- Use the TRAILING_OVERLAP() helper.
- Update changelog text.

Changes in v2:
- Update verdict after (compat_uint_t *)st->data;
- Link: https://lore.kernel.org/linux-hardening/adgL5wPm9VpaV3MO@kspp/

v1:
- Link: https://lore.kernel.org/linux-hardening/adbIKC0cZcK7VcCF@kspp/

 net/netfilter/x_tables.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

index b39017c80548..9f837fb5ceb4 100644
@@ -819,13 +819,17 @@ EXPORT_SYMBOL_GPL(xt_compat_match_to_user);
 
 /* non-compat version may have padding after verdict */
 struct compat_xt_standard_target {
-	struct compat_xt_entry_target t;
-	compat_uint_t verdict;
+	/* Must be last as it ends in a flexible-array member. */
+	TRAILING_OVERLAP(struct compat_xt_entry_target, t, data,
+		compat_uint_t verdict;
+	);
 };
 
 struct compat_xt_error_target {
-	struct compat_xt_entry_target t;
-	char errorname[XT_FUNCTION_MAXNAMELEN];
+	/* Must be last as it ends in a flexible-array member. */
+	TRAILING_OVERLAP(struct compat_xt_entry_target, t, data,
+		char errorname[XT_FUNCTION_MAXNAMELEN];
+	);
 };
 
 int xt_compat_check_entry_offsets(const void *base, const char *elems,

2.43.0