!761 net: stmmac: fix potential double free of dma descriptor resources

Merge Pull Request from: @Hongchen_Zhang 
 
a double free problem in stmmac driver was triggered when doing mugen test,
After apply this patch the double free problem disappear. 
 
Link:https://gitee.com/openeuler/kernel/pulls/761 

Reviewed-by: Jialin Zhang <zhangjialin11@huawei.com> 
Reviewed-by: Guo Dongtai <guodongtai@kylinos.cn> 
Reviewed-by: Yue Haibing <yuehaibing@huawei.com> 
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com> 

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

@@ -1635,10 +1635,15 @@ static void free_dma_rx_desc_resources(struct stmmac_priv *priv)
 			dma_free_coherent(priv->device, priv->dma_rx_size *
 					  sizeof(struct dma_extended_desc),
 					  rx_q->dma_erx, rx_q->dma_rx_phy);
+		rx_q->dma_rx = NULL;
+		rx_q->dma_erx = NULL;
 
 		kfree(rx_q->buf_pool);
+		rx_q->buf_pool = NULL;
+
 		if (rx_q->page_pool)
 			page_pool_destroy(rx_q->page_pool);
+		rx_q->page_pool = NULL;
 	}
 }
 
@@ -1675,8 +1680,15 @@ static void free_dma_tx_desc_resources(struct stmmac_priv *priv)
 
 		dma_free_coherent(priv->device, size, addr, tx_q->dma_tx_phy);
 
+		tx_q->dma_etx = NULL;
+		tx_q->dma_entx = NULL;
+		tx_q->dma_tx = NULL;
+
 		kfree(tx_q->tx_skbuff_dma);
+		tx_q->tx_skbuff_dma = NULL;
+
 		kfree(tx_q->tx_skbuff);
+		tx_q->tx_skbuff = NULL;
 	}
 }