krb5: Use iovecs internally for checksum verification

When verifying checksums, pass iovecs through to the individual
verify routines.

lib/krb5/crypto.h

@@ -103,8 +103,8 @@ struct _krb5_checksum_type {
 				Checksum *csum);
     krb5_error_code (*verify)(krb5_context context,
 			      struct _krb5_key_data *key,
-			      const void *buf, size_t len,
 			      unsigned usage,
+			      const struct krb5_crypto_iov *iov, int niov,
 			      Checksum *csum);
 };