ubifs_hmac_zero
if (ubifs_hmac_zero(c, c->mst_node->hmac)) {
if (ubifs_hmac_zero(c, sup->hmac)) {
if (ubifs_authenticated(c) && ubifs_hmac_zero(c, c->sup_node->hmac)) {
bool ubifs_hmac_zero(struct ubifs_info *c, const u8 *hmac);