bts: Start a DL TBF if needed after establishment of an UL TBF

Currently an existing DL TBF can get lost in the process of
establishing an UL TBF via RACH. This can lead to stalled connections
until the network sends more LLC frames.

This commit adds a check for a non-empty LLC queue after the UL TBF
has been established to rcv_control_ack (GPRS_RLCMAC_UL_ASS_WAIT_ACK
path) to eventually establish a new DL TBF on the UL TBF's PACCH.

Sponsored-by: On-Waves ehf
diff --git a/tests/tbf/TbfTest.cpp b/tests/tbf/TbfTest.cpp
index 2af7e3a..92679bc 100644
--- a/tests/tbf/TbfTest.cpp
+++ b/tests/tbf/TbfTest.cpp
@@ -895,9 +895,8 @@
 	ms = the_bts.ms_by_tlli(tlli1);
 	OSMO_ASSERT(ms2 == ms);
 
-	/* DL TBF should be the same */
-	/* OSMO_ASSERT(ms->dl_tbf()); */
-	/* OSMO_ASSERT(ms->dl_tbf() == dl_tbf); */
+	/* A DL TBF should still exist */
+	OSMO_ASSERT(ms->dl_tbf());
 
 	/* No queued packets should be lost */
 	OSMO_ASSERT(ms->llc_queue()->size() == 2);