make sure subscr->net is always set

since a subscriber is an element of the gsm_network, we have to ensure
subscr->net is always set correctly.  We do this by using gsm_network
as an argument to all functions that resolve or create a subscriber.
diff --git a/openbsc/src/vty_interface.c b/openbsc/src/vty_interface.c
index 4626890..11b2ff6 100644
--- a/openbsc/src/vty_interface.c
+++ b/openbsc/src/vty_interface.c
@@ -589,7 +589,7 @@
 	const char *imsi = argv[0];
 	struct gsm_subscriber *subscr;
 
-	subscr = subscr_get_by_imsi(imsi);
+	subscr = subscr_get_by_imsi(gsmnet, imsi);
 	if (!subscr) {
 		vty_out(vty, "%% No subscriber for IMSI %s%s",
 			imsi, VTY_NEWLINE);
@@ -855,7 +855,7 @@
 
 	if (argc >= 1) {
 		imsi = argv[0];
-		subscr = subscr_get_by_imsi(imsi);
+		subscr = subscr_get_by_imsi(gsmnet, imsi);
 		if (!subscr) {
 			vty_out(vty, "%% unknown subscriber%s",
 				VTY_NEWLINE);