gprs: Add counters related to LLC layer patching

This commit adds the following counters:

  - attach-reqs:   Number of Attach Request messages
  - attach-rejs:   Number of Attach Reject messages
  - tlli-cache:    Size of the TLLI cache

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/gbproxy/gbproxy_test.ok b/openbsc/tests/gbproxy/gbproxy_test.ok
index fb8991c..58f4951 100644
--- a/openbsc/tests/gbproxy/gbproxy_test.ok
+++ b/openbsc/tests/gbproxy/gbproxy_test.ok
@@ -1678,6 +1678,8 @@
     RAID patched              (BSS ): 9
     RAID patched              (SGSN): 3
     APN patched                     : 2
+    Attach Request count            : 1
+    TLLI cache size                 : 2
 --- Bad cases ---
 
 TLLI is already detached, shouldn't patch
@@ -1714,5 +1716,7 @@
     RAID patched              (BSS ): 10
     RAID patched              (SGSN): 3
     APN patched                     : 2
+    Attach Request count            : 1
+    TLLI cache size                 : 2
 ===== GbProxy test END