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/osmo-bsc_nat/bsc_nat.c b/openbsc/src/osmo-bsc_nat/bsc_nat.c
index 6ae8c05..a4dd679 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat.c
@@ -1573,12 +1573,11 @@
 	osmo_timer_schedule(&sccp_close, SCCP_CLOSE_TIME, 0);
 }
 
-extern void *tall_msgb_ctx;
 extern void *tall_ctr_ctx;
 static void talloc_init_ctx()
 {
 	tall_bsc_ctx = talloc_named_const(NULL, 0, "nat");
-	tall_msgb_ctx = talloc_named_const(tall_bsc_ctx, 0, "msgb");
+	msgb_talloc_ctx_init(tall_bsc_ctx, 0);
 	tall_ctr_ctx = talloc_named_const(tall_bsc_ctx, 0, "counter");
 }