[BRIDGE]: random extra bytes on STP TCN packet

We seem to send 3 extra bytes in a TCN, which will be whatever happens
to be on the stack. Thanks to Aji_Srinivas@emc.com for seeing.
Signed-off-by: NStephen Hemminger <shemminger@osdl.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_stp_bpdu.c

@@ -121,7 +121,7 @@ void br_send_tcn_bpdu(struct net_bridge_port *p)
 	buf[1] = 0;
 	buf[2] = 0;
 	buf[3] = BPDU_TYPE_TCN;
-	br_send_bpdu(p, buf, 7);
+	br_send_bpdu(p, buf, 4);
 }
 
 /*