mac80211: add TX fastpath

In order to speed up mac80211's TX path, add the "fast-xmit" cache
that will cache the data frame 802.11 header and other data to be
able to build the frame more quickly. This cache is rebuilt when
external triggers imply changes, but a lot of the checks done per
packet today are simplified away to the check for the cache.

There's also a more detailed description in the code.

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

net/mac80211/ieee80211_i.h

@@ -1651,6 +1651,11 @@ struct sk_buff *
 ieee80211_build_data_template(struct ieee80211_sub_if_data *sdata,
 			      struct sk_buff *skb, u32 info_flags);
 
+void ieee80211_check_fast_xmit(struct sta_info *sta);
+void ieee80211_check_fast_xmit_all(struct ieee80211_local *local);
+void ieee80211_check_fast_xmit_iface(struct ieee80211_sub_if_data *sdata);
+void ieee80211_clear_fast_xmit(struct sta_info *sta);
+
 /* HT */
 void ieee80211_apply_htcap_overrides(struct ieee80211_sub_if_data *sdata,
 				     struct ieee80211_sta_ht_cap *ht_cap);