ul_tbf: Clean up handle_tbf_reject()

Document the function, make it look similar to usual TBF creation path
tbf_alloc_ul()->tbf_alloc_ul_tbf->tbf::setup(), which it mimics with
some differences.
Get rid of unneeded stuff like creating MS and settings its TLLI (that's
already done in only caller of the function). There's no need for
calling update_ms() either.

Change-Id: I61df2e4f0f0df1f8db941741a2d35a2319252c5e
diff --git a/tests/tbf/TbfTest.cpp b/tests/tbf/TbfTest.cpp
index c839ee0..cf8e083 100644
--- a/tests/tbf/TbfTest.cpp
+++ b/tests/tbf/TbfTest.cpp
@@ -3160,7 +3160,8 @@
 	int ts_no = 7;
 	uint8_t trx_no = 0;
 	uint32_t tlli = 0xffeeddcc;
-	struct gprs_rlcmac_ul_tbf *ul_tbf = NULL;
+	struct gprs_rlcmac_ul_tbf *ul_tbf;
+	struct GprsMs *ms;
 
 	fprintf(stderr, "=== start %s ===\n", __func__);
 
@@ -3168,8 +3169,9 @@
 
 	int rc = 0;
 
-	ul_tbf = handle_tbf_reject(bts, NULL, tlli,
-				trx_no, ts_no);
+	ms = bts_alloc_ms(bts, 0, 0);
+	ms_set_tlli(ms, tlli);
+	ul_tbf = handle_tbf_reject(bts, ms, trx_no, ts_no);
 
 	OSMO_ASSERT(ul_tbf != 0);