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/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index 822c359..b4be631 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -163,7 +163,6 @@
 
 /* NSI that BSSGP uses when transmitting on NS */
 extern struct gprs_ns_inst *bssgp_nsi;
-extern void *tall_msgb_ctx;
 
 extern int bsc_vty_go_parent(struct vty *vty);
 
@@ -328,7 +327,7 @@
 
 	srand(time(NULL));
 	tall_bsc_ctx = talloc_named_const(NULL, 0, "osmo_sgsn");
-	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);