mac80211: Do not free net device after it is unregistered.

The error path in ieee80211_register_hw() may call the unregister_netdev()
and right after it - the free_netdev(), which is wrong, since the
unregister releases the device itself.

So the proposed fix is to NULL the local->mdev after unregister is done
and check this before calling free_netdev().

I checked - no code uses the local->mdev after unregister in this error
path (but even if some did this would be a BUG).
Signed-off-by: NPavel Emelyanov <xemul@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/mac80211/main.c

@@ -1766,6 +1766,7 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 fail_rate:
 	ieee80211_debugfs_remove_netdev(IEEE80211_DEV_TO_SUB_IF(local->mdev));
 	unregister_netdevice(local->mdev);
+	local->mdev = NULL;
 fail_dev:
 	rtnl_unlock();
 	sta_info_stop(local);
@@ -1773,8 +1774,10 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 	debugfs_hw_del(local);
 	destroy_workqueue(local->hw.workqueue);
 fail_workqueue:
-	ieee80211_if_free(local->mdev);
-	local->mdev = NULL;
+	if (local->mdev != NULL) {
+		ieee80211_if_free(local->mdev);
+		local->mdev = NULL;
+	}
 fail_mdev_alloc:
 	wiphy_unregister(local->hw.wiphy);
 	return result;