gbproxy: Reset IMSI acquisition within gbproxy_unregister_link_info

Currently then link_info is not cleaned up completely, when
gbproxy_unregister_link_info is called.

This patch adds a function gbproxy_reset_link that must be defined
externally. This is done in gb_proxy.c, where it resets the IMSI
acquisition.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/gbproxy/gbproxy_test.ok b/openbsc/tests/gbproxy/gbproxy_test.ok
index 16e9d48..d0c9d61 100644
--- a/openbsc/tests/gbproxy/gbproxy_test.ok
+++ b/openbsc/tests/gbproxy/gbproxy_test.ok
@@ -5580,7 +5580,7 @@
     Attach Reject count             : 1
     TLLI cache size                 : 1
     TLLI-Cache: 1
-      TLLI 00000000, IMSI 12131415161718, AGE 0, STORED 1, IMSI acquisition in progress, DE-REGISTERED
+      TLLI 00000000, IMSI 12131415161718, AGE 0, DE-REGISTERED
 Gbproxy global:
 Test TLLI info expiry