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/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index 3fb1104..6d49e19 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -209,8 +209,8 @@
 
 }
 
-/* Allocate a new SGSN MM context */
-struct sgsn_mm_ctx *sgsn_mm_ctx_alloc(uint32_t tlli,
+/* Allocate a new SGSN MM context for GERAN_Gb */
+struct sgsn_mm_ctx *sgsn_mm_ctx_alloc_gb(uint32_t tlli,
 					const struct gprs_ra_id *raid)
 {
 	struct sgsn_mm_ctx *ctx;