net: stmmac: Add missing call to dev_kfree_skb()

When RX HW timestamp is enabled and a frame is discarded we are
not freeing the skb but instead only setting to NULL the entry.

Add a call to dev_kfree_skb_any() so that skb entry is correctly
freed.
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -3333,6 +3333,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 				 * them in stmmac_rx_refill() function so that
 				 * device can reuse it.
 				 */
+				dev_kfree_skb_any(rx_q->rx_skbuff[entry]);
 				rx_q->rx_skbuff[entry] = NULL;
 				dma_unmap_single(priv->device,
 						 rx_q->rx_skbuff_dma[entry],