stmmac: protect tx process with lock (V4)

This patch fixes a problem raised on Orly ARM SMP platform
where, in case of fragmented frames, the descriptors
in the TX ring resulted broken. This was due to a missing lock
protection in the tx process.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Tested-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac.h b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
index 1434bdb..50e95d87 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
@@ -70,6 +70,7 @@
 
 	u32 msg_enable;
 	spinlock_t lock;
+	spinlock_t tx_lock;
 	int wolopts;
 	int wolenabled;
 	int wol_irq;