SMS over GSUP: set source_name in GSUP reply messages

For MO-forwardSM and MT-forwardSM request messages, OsmoHLR applies
routing based on the SMSC address for MO or based on the IMSI for MT.
However, reply messages following these requests are routed passively
based on the destination_name IE.  This passive message routing path
requires the source_name IE to be set as well - implement this
source_name setting.

Related: OS#6135
Change-Id: I0b7f4760bdce8a38d43d3860086c6dfb7b390701
diff --git a/src/libmsc/gsm_04_11_gsup.c b/src/libmsc/gsm_04_11_gsup.c
index 6225c6c..d1c2f27 100644
--- a/src/libmsc/gsm_04_11_gsup.c
+++ b/src/libmsc/gsm_04_11_gsup.c
@@ -201,6 +201,7 @@
 	/* Ensure routing through OsmoHLR to the MT-sending SMSC */
 	gsup_msg.destination_name = trans->sms.gsup_source_name;
 	gsup_msg.destination_name_len = trans->sms.gsup_source_name_len;
+	gsup_client_mux_tx_set_source(trans->net->gcm, &gsup_msg);
 
 	return gsup_client_mux_tx(trans->net->gcm, &gsup_msg);
 }
@@ -222,6 +223,7 @@
 	/* Ensure routing through OsmoHLR to the MT-sending SMSC */
 	gsup_msg.destination_name = trans->sms.gsup_source_name;
 	gsup_msg.destination_name_len = trans->sms.gsup_source_name_len;
+	gsup_client_mux_tx_set_source(trans->net->gcm, &gsup_msg);
 
 	/* SM-RP-Cause value */
 	gsup_msg.sm_rp_cause = &cause;