big_keys: Use struct for internal payload

The randstruct GCC plugin gets upset when it sees struct path (which is
randomized) being assigned from a "void *" (which it cannot type-check).

There's no need for these casts, as the entire internal payload use is
following a normal struct layout. Convert the enum-based void * offset
dereferencing to the new big_key_payload struct. No meaningful machine
code changes result after this change, and source readability is improved.

Drop the randstruct exception now that there is no "confusing" cross-type
assignment.

Cc: David Howells <dhowells@redhat.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-hardening@vger.kernel.org
Cc: keyrings@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

scripts/gcc-plugins/randomize_layout_plugin.c

@@ -50,8 +50,6 @@ static const struct whitelist_entry whitelist[] = {
 	{ "drivers/net/ethernet/sun/niu.c", "page", "address_space" },
 	/* unix_skb_parms via UNIXCB() buffer */
 	{ "net/unix/af_unix.c", "unix_skb_parms", "char" },
-	/* big_key payload.data struct splashing */
-	{ "security/keys/big_key.c", "path", "void *" },
 	{ }
 };