BGP: Fixes serious bug in TX handling

Under some circumstances and heavy load, TX could be postponed
until the session fails with hold timer expired.

Thanks to Javor Kliachev for making the bug reproductible.

proto/bgp/packets.c

@@ -769,7 +769,7 @@ bgp_schedule_packet(struct bgp_conn *conn, int type)
 {
   DBG("BGP: Scheduling packet type %d\n", type);
   conn->packets_to_send |= 1 << type;
-  if (conn->sk && conn->sk->tpos == conn->sk->tbuf)
+  if (conn->sk && conn->sk->tpos == conn->sk->tbuf && !ev_active(conn->tx_ev))
     ev_schedule(conn->tx_ev);
 }