net: broadcom: bcm4908enet: Fix a double free in bcm4908_enet_dma_alloc

In bcm4908_enet_dma_alloc, if callee bcm4908_dma_alloc_buf_descs() failed,
it will free the ring->cpu_addr by dma_free_coherent() and return error.
Then bcm4908_enet_dma_free() will be called, and free the same cpu_addr
by dma_free_coherent() again.

My patch set ring->cpu_addr to NULL after it is freed in
bcm4908_dma_alloc_buf_descs() to avoid the double free.

Fixes: 4feffead ("net: broadcom: bcm4908enet: add BCM4908 controller driver")
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bcm4908_enet.c

@@ -172,6 +172,7 @@ static int bcm4908_dma_alloc_buf_descs(struct bcm4908_enet *enet,
 
 err_free_buf_descs:
 	dma_free_coherent(dev, size, ring->cpu_addr, ring->dma_addr);
+	ring->cpu_addr = NULL;
 	return -ENOMEM;
 }