mac80211: fix the skb cloned check in the tx path

Using skb_header_cloned to check if it's safe to write to the skb is not
enough - mac80211 also touches the tailroom of the skb.
Initially this check was only used to increase a counter, however this
commit changed the code to also skip skb data reallocation if no extra
head/tailroom was needed:

commit 4cd06a34
mac80211: skip unnecessary pskb_expand_head calls

It added a regression at least with iwl3945, which is fixed by this patch.
Reported-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Tested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -1547,7 +1547,7 @@ static int ieee80211_skb_resize(struct ieee80211_local *local,
 		skb_orphan(skb);
 	}
 
-	if (skb_header_cloned(skb))
+	if (skb_cloned(skb))
 		I802_DEBUG_INC(local->tx_expand_skb_head_cloned);
 	else if (head_need || tail_need)
 		I802_DEBUG_INC(local->tx_expand_skb_head);