hammer_lock_ex_try
int hammer_lock_ex_try(struct hammer_lock *lock);
if (hammer_lock_ex_try(&parent->lock)) {
if (hammer_lock_ex_try(&child->lock) != 0) {
if (hammer_lock_ex_try(&hmp->volume_lock) != 0) {
if (hammer_lock_ex_try(&hmp->volume_lock) != 0) {
if (hammer_lock_ex_try(&hmp->volume_lock) != 0) {