sgsn: Add gsm0408_gprs_access_cancelled

This function is called to delete an established MM context
silently without invoking a detach procedure.

It is called when a subscriber is cancelled by the HLR. This
generally happens, when an MS has moved to another routing area and
has to use another SGSN.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index 86dd0a2..4b061a5 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -13,6 +13,7 @@
   - Routing Area Update Request (valid)
   - Routing Area Update Request (invalid type)
   - Routing Area Update Request (invalid CAP length)
+Testing cancellation
 Testing P-TMSI allocation
   - sgsn_alloc_ptmsi
   - Repeated Attach Request