net: stmmac: fix completely hung TX when using TSO

stmmac_tso_allocator can fail to set the Last Descriptor bit
on a descriptor that actually was the last descriptor.

This happens when the buffer of the last descriptor ends
up having a size of exactly TSO_MAX_BUFF_SIZE.

When the IP eventually reaches the next last descriptor,
which actually has the bit set, the DMA will hang.

When the DMA hangs, we get a tx timeout, however,
since stmmac does not do a complete reset of the IP
in stmmac_tx_timeout, we end up in a state with
completely hung TX.
Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -2724,7 +2724,7 @@ static void stmmac_tso_allocator(struct stmmac_priv *priv, unsigned int des,
 
 		priv->hw->desc->prepare_tso_tx_desc(desc, 0, buff_size,
 			0, 1,
-			(last_segment) && (buff_size < TSO_MAX_BUFF_SIZE),
+			(last_segment) && (tmp_len <= TSO_MAX_BUFF_SIZE),
 			0, 0);
 
 		tmp_len -= TSO_MAX_BUFF_SIZE;