cfg80211: fix dangling scan request checking

My patch "cfg80211: fix deadlock" broke the code it
was supposed to fix, the scan request checking. But
it's not trivial to put it back the way it was, since
the original patch had a deadlock.

Now do it in a completely new way: queue the check
off to a work struct, where we can freely lock. But
that has some more complications, like needing to
wait for it to be done before the wiphy/rdev can be
destroyed, so some code is required to handle that.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/wireless/core.h

@@ -50,6 +50,8 @@ struct cfg80211_registered_device {
 	struct mutex devlist_mtx;
 	struct list_head netdev_list;
 	int devlist_generation;
+	int opencount; /* also protected by devlist_mtx */
+	wait_queue_head_t dev_wait;
 
 	/* BSSes/scanning */
 	spinlock_t bss_lock;