[PATCH][next] crypto: drbg - 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/drbg.c:1445:27: 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 char ctx[]; onto the FAM struct
shash_desc::__ctx,[] 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/drbg.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/crypto/drbg.c b/crypto/drbg.c
index 511a27c91813..e9f9775c237f 100644
--- a/crypto/drbg.c
+++ b/crypto/drbg.c
@@ -1442,9 +1442,12 @@ static void drbg_kcapi_set_entropy(struct crypto_rng *tfm,
 
 #if defined(CONFIG_CRYPTO_DRBG_HASH) || defined(CONFIG_CRYPTO_DRBG_HMAC)
 struct sdesc {
-	struct shash_desc shash;
-	char ctx[];
+	/* Must be last as it ends in a flexible-array member. */
+	TRAILING_OVERLAP(struct shash_desc, shash, __ctx,
+		char ctx[];
+	);
 };
+static_assert(offsetof(struct sdesc, shash.__ctx) == offsetof(struct sdesc, ctx));
 
 static int drbg_init_hash_kernel(struct drbg_state *drbg)
 {
-- 
2.43.0