rsl: Fix dropping of LAPDm UA message.

In some cases, when successive mobile originated calls are made, the LAPDm UA
message gets lost because the channel is relased to early. Too overcome the
problem we do not send release indications immediately. Instead a flag will be
set and the message stored and sent on the next TCH-RTS-IND.

This commit adds the required flag and the msg-buffer to struct gsm_lchan.
See also coresponding change in osmo-bts.git:
Change-Id Ie4f70c75f0137b4bd72d579b3a32575bac2fca38

This patch is is a slightly improved/reformatted version of:
95d1f15ad1

Change-Id: I15fc1ef8e9e83f009bde96de9a8e95702cffbce6

openbsc/include/openbsc/gsm_data_shared.h

@@ -350,6 +350,8 @@ struct gsm_lchan {
 		uint8_t current;
 		uint8_t fixed;
 	} ms_power_ctrl;
+
+	struct msgb *pending_rel_ind_msg;
 #endif
 };