sms: Increment the RP Message Reference for each transaction

Each RP-DATA should have a unique msg reference. Currently 42 is
used for all of these. Remember the last reference we used and
increment it on the next SMS. Do not track if the reference is
still in use a clash is a lot less likely now. First unless SMPP
is used only one SMS is delivered at a time, second the transaction
space is a lot smaller than the one for the reference.
diff --git a/openbsc/src/libmsc/gsm_04_11.c b/openbsc/src/libmsc/gsm_04_11.c
index 97a67ee..45e5dc5 100644
--- a/openbsc/src/libmsc/gsm_04_11.c
+++ b/openbsc/src/libmsc/gsm_04_11.c
@@ -450,7 +450,7 @@
 	rp = (struct gsm411_rp_hdr *)msgb_push(msg, sizeof(*rp));
 	rp->len = len + 2;
 	rp->msg_type = rp_msg_type;
-	rp->msg_ref = rp_msg_ref; /* FIXME: Choose randomly */
+	rp->msg_ref = rp_msg_ref;
 
 	return gsm411_smr_send(inst, rl_msg_type, msg);
 }
@@ -836,7 +836,7 @@
 	struct msgb *msg = gsm411_msgb_alloc();
 	struct gsm_trans *trans;
 	uint8_t *data, *rp_ud_len;
-	uint8_t msg_ref = 42;
+	uint8_t msg_ref = sms_next_rp_msg_ref(conn);
 	int transaction_id;
 	int rc;