batman-adv: Detect missing primaryif during tp_send as error

The throughput meter detects different situations as problems for the
current test. It stops the test after these and reports it to userspace.
This also has to be done when the primary interface disappeared during the
test.

Fixes: 33a3bb4a ("batman-adv: throughput meter implementation")
Reported-by: NJoe Perches <joe@perches.com>
Signed-off-by: NSven Eckelmann <sven@narfation.org>
Signed-off-by: NSimon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/tp_meter.c

@@ -837,6 +837,7 @@ static int batadv_tp_send(void *arg)
 	primary_if = batadv_primary_if_get_selected(bat_priv);
 	if (unlikely(!primary_if)) {
 		err = BATADV_TP_REASON_DST_UNREACHABLE;
+		tp_vars->reason = err;
 		goto out;
 	}