cfg80211: fix rcu in cfg80211_unregister_wdev

Callers of cfg80211_unregister_wdev can free the wdev object
immediately after this function returns. This may crash the kernel
because this wdev object is still in use by other threads.
Add synchronize_rcu() after list_del_rcu to make sure wdev object can
be safely freed.
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>

net/wireless/core.c

@@ -1012,6 +1012,7 @@ void cfg80211_unregister_wdev(struct wireless_dev *wdev)
 	nl80211_notify_iface(rdev, wdev, NL80211_CMD_DEL_INTERFACE);
 
 	list_del_rcu(&wdev->list);
+	synchronize_rcu();
 	rdev->devlist_generation++;
 
 	switch (wdev->iftype) {