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/libcommon/talloc_ctx.c b/openbsc/src/libcommon/talloc_ctx.c
index a917a8c..7a8bfd3 100644
--- a/openbsc/src/libcommon/talloc_ctx.c
+++ b/openbsc/src/libcommon/talloc_ctx.c
@@ -20,7 +20,6 @@
 #include <osmocom/core/talloc.h>
 
 extern void *tall_bsc_ctx;
-extern void *tall_msgb_ctx;
 extern void *tall_fle_ctx;
 extern void *tall_locop_ctx;
 extern void *tall_authciphop_ctx;
@@ -38,7 +37,7 @@
 
 void talloc_ctx_init(void *ctx_root)
 {
-	tall_msgb_ctx = talloc_named_const(ctx_root, 0, "msgb");
+	msgb_talloc_ctx_init(ctx_root, 0);
 	tall_fle_ctx = talloc_named_const(ctx_root, 0, "bs11_file_list_entry");
 	tall_locop_ctx = talloc_named_const(ctx_root, 0, "loc_updating_oper");
 	tall_authciphop_ctx = talloc_named_const(ctx_root, 0, "auth_ciph_oper");