Cycles: Use lock in the memory statistics

CPU rendering is allowed to allocate memory from multiple threads,
which means statistics need to be avare of this.

intern/cycles/util/util_atomic.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2014 Blender Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License
+ */
+
+#ifndef __UTIL_ATOMIC_H__
+#define __UTIL_ATOMIC_H__
+
+/* Using atomic ops header from Blender. */
+#include "atomic_ops.h"
+
+ATOMIC_INLINE void atomic_update_max_z(size_t *maximum_value, size_t value)
+{
+	size_t prev_value = *maximum_value;
+	while (prev_value < value) {
+		if (atomic_cas_z(maximum_value, prev_value, value) != prev_value) {
+			break;
+		}
+	}
+}
+
+#endif /* __UTIL_ATOMIC_H__ */