stp: stop treating a port as an edge once it hears a BPDU

802.1D has a port leave the edge state when a BPDU arrives on it. Here
the flag was only ever cleared by the loop latch, root guard, a link
coming back, "stp on", "stp off" and the edge command itself, so a port
that auto-edged during the three seconds of silence after link-up kept
the flag for as long as it stayed up, whatever the neighbour sent.

Two things read that flag. The status page prints it, so a port talking
to a bridge reported edge 1 and there was no way to tell from the output
whether a BPDU had ever arrived. More quietly, stp_topology_change()
returns early for an edge port, which is right for a real one and wrong
for this: a topology change on such a port was neither counted nor
propagated, and port_l2_forget_port() never ran, so what was learned
behind it stayed in the table.

Only the flag is cleared. The port is not pushed back through the listen
period, which would take a working link out of forwarding for a forward
delay the first time a neighbour speaks.

rtl837x_stp.c

@@ -451,6 +451,16 @@ void stp_in(void) __banked
 
 	stp_bpdu_age[port] = 0;
 
+	/* A port that hears a BPDU is not an edge port, whatever it decided
+	 * during the silence after the link came up. Only the flag is dropped:
+	 * the port keeps whatever forwarding state the rules below give it,
+	 * rather than being pushed back through the listen period, which would
+	 * black-hole a working link for a forward delay on the first BPDU. The
+	 * flag matters beyond the status page, since stp_topology_change()
+	 * exempts edge ports and so would go on skipping the counter and the
+	 * L2 flush for a port that has a bridge behind it. */
+	stp_pflags[port] &= ~STP_PF_OPEREDGE;
+
 	if (STP_I->bpdu_type == 0x80) {
 		/* TCN: a downstream bridge reports a topology change. Acknowledge it
 		 * on this port so the sender stops repeating; the change itself is