crypto: chacha20poly1305 - Annotate struct chachapoly_ctx with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
salt to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/chacha20poly1305.c

@@ -27,7 +27,7 @@ struct chachapoly_ctx {
 	struct crypto_ahash *poly;
 	/* key bytes we use for the ChaCha20 IV */
 	unsigned int saltlen;
-	u8 salt[];
+	u8 salt[] __counted_by(saltlen);
 };
 
 struct poly_req {