msgb ctx: use new msgb_talloc_ctx_init(), don't access talloc_msgb_ctx

Drop extern definitions of talloc_msgb_ctx and use msgb_talloc_ctx_init()
instead.

In sgsn_test.c, use a local variable msgb_ctx to do the talloc report
from the return value of msgb_talloc_ctx_init().

Change-Id: I2f9ace855f0ecbdc9adf5d75bcb1a3d666570de4
diff --git a/openbsc/src/gprs/gb_proxy_main.c b/openbsc/src/gprs/gb_proxy_main.c
index 2ddfca7..04875c7 100644
--- a/openbsc/src/gprs/gb_proxy_main.c
+++ b/openbsc/src/gprs/gb_proxy_main.c
@@ -189,8 +189,6 @@
 	}
 }
 
-extern void *tall_msgb_ctx;
-
 extern int bsc_vty_go_parent(struct vty *vty);
 
 static struct vty_app_info vty_info = {
@@ -231,7 +229,7 @@
 	int rc;
 
 	tall_bsc_ctx = talloc_named_const(NULL, 0, "nsip_proxy");
-	tall_msgb_ctx = talloc_named_const(tall_bsc_ctx, 0, "msgb");
+	msgb_talloc_ctx_init(tall_bsc_ctx, 0);
 
 	signal(SIGINT, &signal_handler);
 	signal(SIGABRT, &signal_handler);