gprs/sgsn: rename sgsn_mm_ctx_alloc() -> sgsn_mm_ctx_alloc_gb()

Postfix the ran type to clarify the purpose.
Because of the new support of the Iu ran type, there are 2 functions to allocate a mm ctx.
For Iu it's sgsn_mm_ctx_alloc_iu(). For gb it should be named in the same way.

Change-Id: Ic49009e8c20c12308855e1409c09004698c79b95
diff --git a/openbsc/tests/sgsn/sgsn_test.c b/openbsc/tests/sgsn/sgsn_test.c
index bcda9a0..0aa142f 100644
--- a/openbsc/tests/sgsn/sgsn_test.c
+++ b/openbsc/tests/sgsn/sgsn_test.c
@@ -190,7 +190,7 @@
 	int old_count = count(gprs_llme_list());
 
 	lle = gprs_lle_get_or_create(tlli, 3);
-	ctx = sgsn_mm_ctx_alloc(tlli, raid);
+	ctx = sgsn_mm_ctx_alloc_gb(tlli, raid);
 	ctx->gmm_state = GMM_REGISTERED_NORMAL;
 	ctx->gb.llme = lle->llme;