cfg80211/wext: fix message ordering

cfg80211/wext: fix message ordering

commit cb150b9d23be6ee7f3a0fff29784f1c5b5ac514d upstream.

Since cfg80211 frequently takes actions from its netdev notifier
call, wireless extensions messages could still be ordered badly
since the wext netdev notifier, since wext is built into the
kernel, runs before the cfg80211 netdev notifier. For example,
the following can happen:

5: wlan1: <BROADCAST,MULTICAST> mtu 1500 qdisc mq state DOWN group default
    link/ether 02:00:00:00:01:00 brd ff:ff:ff:ff:ff:ff
5: wlan1: <BROADCAST,MULTICAST,UP>
    link/ether

when setting the interface down causes the wext message.

To also fix this, export the wireless_nlevent_flush() function
and also call it from the cfg80211 notifier.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[bwh: Backported to 3.2:
 - Add default case in cfg80211_netdev_notifier_call() which bypasses
   the added wireless_nlevent_flush() (added upstream by commit
   6784c7db8d43 "cfg80211: change return value of notifier function")
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>