staging: wfx: fix potential use-after-free

wfx_tx_policy_put() use data from the skb. However, the call to
skb_pull() has just discarded them (even if the memory is in fact not
really discarded).

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200515083325.378539-11-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/data_tx.c

@@ -494,8 +494,8 @@ static void wfx_skb_dtor(struct wfx_vif *wvif, struct sk_buff *skb)
 			      req->data_flags.fc_offset;
 
 	WARN_ON(!wvif);
-	skb_pull(skb, offset);
 	wfx_tx_policy_put(wvif, req->tx_flags.retry_policy_index);
+	skb_pull(skb, offset);
 	ieee80211_tx_status_irqsafe(wvif->wdev->hw, skb);
 }