mac80211: avoid duplicate TX path station lookup

Instead of looking up the destination station twice in the TX path
(first to build the header, and then for control processing), save
it when building the header and use it later in the TX path.

To avoid having to look up the station in the many callers, allow
those to pass %NULL which keeps the existing lookup.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/sta_info.c

@@ -1231,7 +1231,7 @@ static void ieee80211_send_null_response(struct ieee80211_sub_if_data *sdata,
 	}
 
 	info->band = chanctx_conf->def.chan->band;
-	ieee80211_xmit(sdata, skb);
+	ieee80211_xmit(sdata, sta, skb);
 	rcu_read_unlock();
 }