mac80211: use skb list for fragments

We are currently linking the skbs by using skb->next
directly. This works, but the preferred way is to use
a struct sk_buff_head instead. That also prepares for
passing that to drivers directly.

While at it I noticed we calculate the duration for
fragments twice -- remove one of them.

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

net/mac80211/ieee80211_i.h

@@ -142,6 +142,7 @@ typedef unsigned __bitwise__ ieee80211_tx_result;
 
 struct ieee80211_tx_data {
 	struct sk_buff *skb;
+	struct sk_buff_head skbs;
 	struct ieee80211_local *local;
 	struct ieee80211_sub_if_data *sdata;
 	struct sta_info *sta;