mgcp: Remember the initial sequence number of the stream

This information is required for calculating the packet loss at the
end of a phone call.

openbsc/include/openbsc/mgcp_internal.h

@@ -46,6 +46,7 @@ struct mgcp_rtp_state {
 
 	uint32_t orig_ssrc;
 	uint32_t ssrc;
+	int first_seq_no;
 	uint16_t seq_no;
 	int seq_offset;
 	uint32_t last_timestamp;

openbsc/src/libmgcp/mgcp_network.c

@@ -120,6 +120,7 @@ static void patch_and_count(struct mgcp_endpoint *endp, struct mgcp_rtp_state *s
 	timestamp = ntohl(rtp_hdr->timestamp);
 
 	if (!state->initialized) {
+		state->first_seq_no = seq;
 		state->seq_no = seq - 1;
 		state->ssrc = state->orig_ssrc = rtp_hdr->ssrc;
 		state->initialized = 1;