stp: pass a topology change on only from a forwarding port

stp_in() propagated the TC flag of any BPDU that did not arrive on an
alternate port, and acknowledged and propagated a TCN from any port, so a
port still in its listen period or held discarding in a dispute could flush
the addresses of every other port and move the topology change counter.
802.1D-2004 17.31 keeps the Topology Change machine in LEARNING until the
port forwards and drops rcvdTc and rcvdTcn there. Both paths now ask for
the forwarding state first; an alternate port is discarding, so the old
check is covered by the new one.

Scenario 53 plays a TC flag and a TCN into a port that is still discarding,
and then into the same port once it forwards. Scenario 51 used a TCN on a
port one tick short of its edge transition as the trigger for the clock it
tests; it now waits for the transition.

rtl837x_stp.c

@@ -1018,6 +1018,8 @@ void stp_in(void) __banked
 		return;
 
 	if (STP_I->bpdu_type == BPDU_TYPE_TCN) {
+		if (stp_state_get(port) != 0b11)
+			return;
 		stp_tx_flags_extra = BPDU_FLAG_TCACK;
 		stp_cnf_send(port);
 		uip_len = 0;
@@ -1063,7 +1065,7 @@ void stp_in(void) __banked
 
 	if ((STP_I->flags & BPDU_FLAG_TCACK) && port == stp_root_port)
 		stp_tcwhile[port] = 0;
-	if ((STP_I->flags & BPDU_FLAG_TC) && !((stp_alt >> port) & 1) && stp_tc_prop(port))
+	if ((STP_I->flags & BPDU_FLAG_TC) && stp_state_get(port) == 0b11 && stp_tc_prop(port))
 		stp_tc_count++;
 
 	if ((stp_pflags[port] & STP_PF_ROOTGUARD)