lapdm: Track fn of primitives in struct lapdm_msg_ctx

This field will be used in follow-up commits to provide FN information
in RSLms primitives towars upper layers. This is needed for instance on
the MS side when a CCCH_DATA.ind is received containing a TBF ImmAss
with a relative FN indicating the Starting Time. Without tracking FN
advance, the uppers layers are not capable of figuring out the absolute
FN of the TBF Starting time.

The struct lapdm_msg_ctx is not really used outside of libosmocore, so
we are safe extending it.

Related: OS#3626
Change-Id: Icf986f4202703eb452bedc1b749bb8ce0c73706f
diff --git a/include/osmocom/gsm/lapdm.h b/include/osmocom/gsm/lapdm.h
index 0e99743..1a39fca 100644
--- a/include/osmocom/gsm/lapdm.h
+++ b/include/osmocom/gsm/lapdm.h
@@ -24,6 +24,7 @@
 	uint8_t link_id;
 	uint8_t ta_ind;		/* TA indicated by network */
 	uint8_t tx_power_ind;	/* MS power indicated by network */
+	uint32_t fn;
 };
 
 /*! LAPDm datalink like TS 04.06 / Section 3.5.2 */