net/x25: handle call collisions

If a session in X25_STATE_1 (Awaiting Call Accept) receives a call
request, the session will be closed (x25_disconnect), cause=0x01
(Number Busy) and diag=0x48 (Call Collision) will be set and a clear
request will be send.

Signed-off-by: Martin Schiller <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/x25/x25_in.c

···
 			sk->sk_state_change(sk);
 		break;
 	}
+	case X25_CALL_REQUEST:
+		/* call collision */
+		x25->causediag.cause      = 0x01;
+		x25->causediag.diagnostic = 0x48;
+
+		x25_write_internal(sk, X25_CLEAR_REQUEST);
+		x25_disconnect(sk, EISCONN, 0x01, 0x48);
+		break;
+
 	case X25_CLEAR_REQUEST:
 		if (!pskb_may_pull(skb, X25_STD_MIN_LEN + 2))
 			goto out_clear;