debug heap: implement malloc_usable_size

Change-Id: I49d7918df00cca4b889f6a3cf52c3f903ee5fa2a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2179
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/system/libroot/posix/malloc_debug/malloc_debug_api.cpp

@@ -302,3 +302,21 @@ posix_memalign(void **pointer, size_t alignment, size_t size)
 
 	return 0;
 }
+
+
+extern "C" size_t
+malloc_usable_size(void *ptr)
+{
+	size_t size;
+	thread_id thread;
+
+	if (ptr == NULL)
+		return 0;
+
+	status_t res = sCurrentHeap->get_allocation_info(ptr, &size, &thread);
+
+	if (res != B_OK)
+		return 0;
+
+	return size;
+}