use pthread_cleanup to acquire and release lock

In chunk_prepare(), if multiple threads wait for acquiring f->lock,
and then pthread_cancel() is called, the waiting threads are never
canceled because pthread_mutex_lock() is not a cancellation point.
So, use pthread_cleanup_push/pop to release the lock.

src/main.c

@@ -507,9 +507,9 @@ void *mscp_copy_thread(void *arg)
 	pthread_cleanup_push(mscp_copy_thread_cleanup, t);
 
 	while (1) {
-		lock_acquire(&m.chunk_lock);
+		LOCK_ACQUIRE_THREAD(&m.chunk_lock);
 		c = chunk_acquire(&m.chunk_list);
-		lock_release(&m.chunk_lock);
+		LOCK_RELEASE_THREAD();
 
 		if (!c)
 			break; /* no more chunks */