llc: Keep track of the number of stored LLC octets

To get the number of LLC octets that are stored in the queue, this
commit adds a m_queue_octets member along with a octets() method.
This value is updated similarly to m_queue_size on each modifying
method call.

Sponsored-by: On-Waves ehf
diff --git a/tests/llc/LlcTest.cpp b/tests/llc/LlcTest.cpp
index 9e1d35d..5cc6e6e 100644
--- a/tests/llc/LlcTest.cpp
+++ b/tests/llc/LlcTest.cpp
@@ -102,24 +102,31 @@
 
 	queue.init();
 	OSMO_ASSERT(queue.size() == 0);
+	OSMO_ASSERT(queue.octets() == 0);
 
 	enqueue_data(&queue, "LLC message");
 	OSMO_ASSERT(queue.size() == 1);
+	OSMO_ASSERT(queue.octets() == 11);
 
 	enqueue_data(&queue, "other LLC message");
 	OSMO_ASSERT(queue.size() == 2);
+	OSMO_ASSERT(queue.octets() == 28);
 
 	dequeue_and_check(&queue, "LLC message");
 	OSMO_ASSERT(queue.size() == 1);
+	OSMO_ASSERT(queue.octets() == 17);
 
 	dequeue_and_check(&queue, "other LLC message");
 	OSMO_ASSERT(queue.size() == 0);
+	OSMO_ASSERT(queue.octets() == 0);
 
 	enqueue_data(&queue, "LLC");
 	OSMO_ASSERT(queue.size() == 1);
+	OSMO_ASSERT(queue.octets() == 3);
 
 	queue.clear(NULL);
 	OSMO_ASSERT(queue.size() == 0);
+	OSMO_ASSERT(queue.octets() == 0);
 
 	printf("=== end %s ===\n", __func__);
 }