mac80211: significantly reduce the cpu load caused by excessive skb header cloning

SVN-Revision: 24669

package/mac80211/patches/540-mac80211_tx_optimization.patch

@@ -0,0 +1,14 @@
+--- a/net/mac80211/tx.c
++++ b/net/mac80211/tx.c
+@@ -1549,8 +1549,10 @@ static int ieee80211_skb_resize(struct i
+ 
+ 	if (skb_header_cloned(skb))
+ 		I802_DEBUG_INC(local->tx_expand_skb_head_cloned);
+-	else
++	else if (head_need || tail_need)
+ 		I802_DEBUG_INC(local->tx_expand_skb_head);
++	else
++		return;
+ 
+ 	if (pskb_expand_head(skb, head_need, tail_need, GFP_ATOMIC)) {
+ 		wiphy_debug(local->hw.wiphy,