stmmac: fix stmmac_tx_avail should be called with TX locked

stmmac_tx_avail() may lie if used unprotected. It's using cur_tx
and dirty_tx index. These index may be already in use by tx_clean
when entering xmit routine. So, this should be called locked.

This can cause transmit queue to be stuck, with following message:
NETDEV WATCHDOG: eth0 (stmmaceth): transmit queue 0 timed out
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: NGiuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -1894,7 +1894,10 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 	unsigned int nopaged_len = skb_headlen(skb);
 	unsigned int enh_desc = priv->plat->enh_desc;
 
+	spin_lock(&priv->tx_lock);
+
 	if (unlikely(stmmac_tx_avail(priv) < nfrags + 1)) {
+		spin_unlock(&priv->tx_lock);
 		if (!netif_queue_stopped(dev)) {
 			netif_stop_queue(dev);
 			/* This is a hard error, log it. */
@@ -1903,8 +1906,6 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		return NETDEV_TX_BUSY;
 	}
 
-	spin_lock(&priv->tx_lock);
-
 	if (priv->tx_path_in_lpi_mode)
 		stmmac_disable_eee_mode(priv);