mac80211: Properly WARN on HW scan before restart

The following race was possible:

1. The device driver requests HW restart.
2. A scan is requested from user space and is propagated
   to the driver. During this flow HW_SCANNING flag is set.
3. The thread that handles the HW restart is scheduled,
   and before starting the actual reconfiguration it
   checks that HW_SCANNING is not set. The flow does so
   without acquiring any lock, and thus the WARN fires.

Fix this by checking that HW_SCANNING is on only after RTNL is
acquired, i.e., user space scan request handling is no longer
in transit.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210618133832.8238ab3e19ab.I2693c581c70251472b4f9089e37e06fb2c18268f@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/main.c

@@ -257,14 +257,13 @@ static void ieee80211_restart_work(struct work_struct *work)
 	/* wait for scan work complete */
 	flush_workqueue(local->workqueue);
 	flush_work(&local->sched_scan_stopped_work);
+	flush_work(&local->radar_detected_work);
+
+	rtnl_lock();
 
 	WARN(test_bit(SCAN_HW_SCANNING, &local->scanning),
 	     "%s called with hardware scan in progress\n", __func__);
 
-	flush_work(&local->radar_detected_work);
-	/* we might do interface manipulations, so need both */
-	rtnl_lock();
-	wiphy_lock(local->hw.wiphy);
 	list_for_each_entry(sdata, &local->interfaces, list) {
 		/*
 		 * XXX: there may be more work for other vif types and even