mac80211: fix station inactive_time shortly after boot

In the first 5 minutes after boot (time of INITIAL_JIFFIES),
ieee80211_sta_last_active() returns zero if last_ack is zero. This
leads to "inactive time" showing jiffies_to_msecs(jiffies).

 # iw wlan0 station get fc:ec:da:64:a6:dd
 Station fc:ec:da:64:a6:dd (on wlan0)
	inactive time:	4294894049 ms
	.
	.
	connected time:	70 seconds

Fix by returning last_rx if last_ack == 0.

Signed-off-by: Ahmed Zaki <anzaki@gmail.com>
Link: https://lore.kernel.org/r/20191031121243.27694-1-anzaki@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/sta_info.c

@@ -2457,7 +2457,8 @@ unsigned long ieee80211_sta_last_active(struct sta_info *sta)
 {
 	struct ieee80211_sta_rx_stats *stats = sta_get_last_rx_stats(sta);
 
-	if (time_after(stats->last_rx, sta->status_stats.last_ack))
+	if (!sta->status_stats.last_ack ||
+	    time_after(stats->last_rx, sta->status_stats.last_ack))
 		return stats->last_rx;
 	return sta->status_stats.last_ack;
 }