[PATCH][next] KEYS: Avoid -Wflex-array-member-not-at-end warning

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

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

Use the new TRAILING_OVERLAP() helper to fix the following warning:

crypto/asymmetric_keys/restrict.c:20:34: 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 MEMBER unsigned char data[10]; onto the FAM
struct asymmetric_key_id::data[], while keeping the FAM and the start
of MEMBER aligned.

The static_assert() ensures this alignment remains, and it's
intentionally placed inmediately after the corresponding structures --no
blank line in between.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 crypto/asymmetric_keys/restrict.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/crypto/asymmetric_keys/restrict.c b/crypto/asymmetric_keys/restrict.c
index afcd4d101ac5..86292965f493 100644
--- a/crypto/asymmetric_keys/restrict.c
+++ b/crypto/asymmetric_keys/restrict.c
@@ -17,9 +17,12 @@ static struct asymmetric_key_id *ca_keyid;
 
 #ifndef MODULE
 static struct {
-	struct asymmetric_key_id id;
-	unsigned char data[10];
+	/* Must be last as it ends in a flexible-array member. */
+	TRAILING_OVERLAP(struct asymmetric_key_id, id, data,
+		unsigned char data[10];
+	);
 } cakey;
+static_assert(offsetof(typeof(cakey), id.data) == offsetof(typeof(cakey), data));
 
 static int __init ca_keys_setup(char *str)
 {
-- 
2.43.0