regulatory: use RCU to protect global and wiphy regdomains

To simplify the locking and not require cfg80211_mutex
(which nl80211 uses to access the global regdomain) and
also to make it possible for drivers to access their
wiphy->regd safely, use RCU to protect these pointers.

Acked-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -2369,7 +2369,7 @@ struct wiphy {
 
 	/* fields below are read-only, assigned by cfg80211 */
 
-	const struct ieee80211_regdomain *regd;
+	const struct ieee80211_regdomain __rcu *regd;
 
 	/* the item in /sys/class/ieee80211/ points to this,
 	 * you need use set_wiphy_dev() (see below) */