pkcs7: fix check for self-signed certificate

pkcs7_validate_trust_one() used 'x509->next == x509' to identify a
self-signed certificate.  That's wrong; ->next is simply the link in the
linked list of certificates in the PKCS#7 message.  It should be
checking ->signer instead.  Fix it.

Fortunately this didn't actually matter because when we re-visited
'x509' on the next iteration via 'x509->signer', it was already seen and
not verified, so we returned -ENOKEY anyway.
Signed-off-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Reviewed-by: NJames Morris <james.l.morris@oracle.com>

crypto/asymmetric_keys/pkcs7_trust.c

@@ -69,7 +69,7 @@ static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
 		 /* Self-signed certificates form roots of their own, and if we
 		  * don't know them, then we can't accept them.
 		  */
-		if (x509->next == x509) {
+		if (x509->signer == x509) {
 			kleave(" = -ENOKEY [unknown self-signed]");
 			return -ENOKEY;
 		}