mac80211: dereference RCU protected probe_resp pointer correctly

This fixes a sparse warning:

cfg.c:502:13: warning: incorrect type in assignment (different address spaces)
cfg.c:502:13:    expected struct sk_buff *old
cfg.c:502:13:    got struct sk_buff [noderef] <asn:4>*probe_resp
Reported-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NArik Nemtsov <arik@wizery.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/cfg.c

@@ -509,7 +509,7 @@ static int ieee80211_set_probe_resp(struct ieee80211_sub_if_data *sdata,
 	if (!resp || !resp_len)
 		return -EINVAL;
 
-	old = sdata->u.ap.probe_resp;
+	old = rtnl_dereference(sdata->u.ap.probe_resp);
 
 	new = dev_alloc_skb(resp_len);
 	if (!new)