gprs_rlcmac_sched: fix incorrect length for CTR_RLC_DL_BYTES

msg->data_len is the total number of bytes available in the buffer,
while for CTR_RLC_DL_BYTES we need to count size of the actual
payload within the buffer.  A consequence of this bug: osmo-pcu
was counting more Downlink bytes than it's actually transmitted.

Change-Id: I6884d220f3d06a79b16c18ccc2d2a6cd047b8251

src/gprs_rlcmac_sched.cpp

@@ -506,7 +506,7 @@ int gprs_rlcmac_rcv_rts_block(struct gprs_rlcmac_bts *bts,
 	}
 
 	/* msg is now available */
-	bts_do_rate_ctr_add(bts, CTR_RLC_DL_BYTES, msg->data_len);
+	bts_do_rate_ctr_add(bts, CTR_RLC_DL_BYTES, msgb_length(msg));
 
 	/* set USF */
 	OSMO_ASSERT(msgb_length(msg) > 0);