mac80211: add separate last_ack variable

Instead of touching the rx_stats.last_rx from the status path, introduce
and use a status_stats.last_ack variable. This will make rx_stats.last_rx
indicate when the last frame was received, making it available for real
"last_rx" and statistics gathering; statistics, when done per-CPU, will
need to figure out which place was updated last for those items where the
"last" value is exposed.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/sta_info.h

@@ -474,6 +474,7 @@ struct sta_info {
 		unsigned long last_tdls_pkt_time;
 		u64 msdu_retries[IEEE80211_NUM_TIDS + 1];
 		u64 msdu_failed[IEEE80211_NUM_TIDS + 1];
+		unsigned long last_ack;
 	} status_stats;
 
 	/* Updated from TX path only, no locking requirements */
@@ -680,4 +681,6 @@ void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta);
 void ieee80211_sta_ps_deliver_poll_response(struct sta_info *sta);
 void ieee80211_sta_ps_deliver_uapsd(struct sta_info *sta);
 
+unsigned long ieee80211_sta_last_active(struct sta_info *sta);
+
 #endif /* STA_INFO_H */