mac80211: fix double-start of remain-on-channel

When a remain-on-channel item is deleted, we remove it
from the list and then start the next item. However,
if it wasn't actually the first item then calling
ieee80211_start_next_roc() is wrong as it will start
the first item -- even if that was already started.

Fix the two places that do this and add a warning to
prevent the problem from reoccurring.
Reported-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/cfg.c

@@ -2362,7 +2362,8 @@ static int ieee80211_cancel_roc(struct ieee80211_local *local,
 
 		list_del(&found->list);
 
-		ieee80211_start_next_roc(local);
+		if (found->started)
+			ieee80211_start_next_roc(local);
 		mutex_unlock(&local->mtx);
 
 		ieee80211_roc_notify_destroy(found);

net/mac80211/offchannel.c

@@ -262,6 +262,9 @@ void ieee80211_start_next_roc(struct ieee80211_local *local)
 	roc = list_first_entry(&local->roc_list, struct ieee80211_roc_work,
 			       list);
 
+	if (WARN_ON_ONCE(roc->started))
+		return;
+
 	if (local->ops->remain_on_channel) {
 		int ret, duration = roc->duration;
 
@@ -377,7 +380,8 @@ void ieee80211_sw_roc_work(struct work_struct *work)
 
 		ieee80211_recalc_idle(local);
 
-		ieee80211_start_next_roc(local);
+		if (roc->started)
+			ieee80211_start_next_roc(local);
 	}
 
  out_unlock: