llc: Move storage of timestamps into gprs_llc_queue

Currently the receive and expiry timestamps are prepended to the LLC
msgb before it is passed to gprs_llc_queue::enqueue(). Since this meta
information should not be counted as LLC octets, the gprs_llc_queue
needs to known about this (unless the correction was done in the LLC
layer).

This commit moves the meta information storage code into
gprs_llc_queue.  The meta data is now stored in the control block
(cb) area of the msgb.

Note that the info pointer that is returned from the dequeue method
is only valid if that method returns a (non-NULL) msgb. It must not
be used after that msgb has been modified or freed.

Sponsored-by: On-Waves ehf
diff --git a/tests/llc/LlcTest.ok b/tests/llc/LlcTest.ok
index 79e23cd..8a5f235 100644
--- a/tests/llc/LlcTest.ok
+++ b/tests/llc/LlcTest.ok
@@ -1,2 +1,4 @@
 === start test_llc_queue ===
 === end test_llc_queue ===
+=== start test_llc_meta ===
+=== end test_llc_meta ===