subscr: Stop all paging requests if one is failing.

If one paging request is timing out the others will timeout soon
as well. With the current code the next timeout would expire the
next request in the queue. We will now stop all paging requests
and then issue a next paging request. So for both paging success
and failure we will now stop all the other requests.

This is mostly a workaround, one should count on how many
BTSes we are paging and wait for all failures before we remove
the item from the queue.
diff --git a/openbsc/tests/channel/channel_test.c b/openbsc/tests/channel/channel_test.c
index a924e5b..a63d8a5 100644
--- a/openbsc/tests/channel/channel_test.c
+++ b/openbsc/tests/channel/channel_test.c
@@ -77,6 +77,7 @@
 void sms_alloc() {}
 void gsm_net_update_ctype(struct gsm_network *network) {}
 void gsm48_secure_channel() {}
+void paging_request_stop() {}
 
 
 struct tlv_definition nm_att_tlvdef;