tg3: avoid double-freeing of rx data memory

If build_skb fails the memory associated with the ring buffer is freed but
the ri->data member is not zeroed in this case. This causes a double-free
of this memory in tg3_free_rings->... path. The patch moves this block after
setting ri->data to NULL.
It would be nice to fix this bug also in stable >= v3.4 trees.

Cc: Nithin Nayak Sujir <nsujir@broadcom.com>
Cc: Michael Chan <mchan@broadcom.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/tg3.c

@@ -6862,19 +6862,19 @@ static int tg3_rx(struct tg3_napi *tnapi, int budget)
 			pci_unmap_single(tp->pdev, dma_addr, skb_size,
 					 PCI_DMA_FROMDEVICE);
 
-			skb = build_skb(data, frag_size);
-			if (!skb) {
-				tg3_frag_free(frag_size != 0, data);
-				goto drop_it_no_recycle;
-			}
-			skb_reserve(skb, TG3_RX_OFFSET(tp));
 			/* Ensure that the update to the data happens
 			 * after the usage of the old DMA mapping.
 			 */
 			smp_wmb();
 
 			ri->data = NULL;
 
+			skb = build_skb(data, frag_size);
+			if (!skb) {
+				tg3_frag_free(frag_size != 0, data);
+				goto drop_it_no_recycle;
+			}
+			skb_reserve(skb, TG3_RX_OFFSET(tp));
 		} else {
 			tg3_recycle_rx(tnapi, tpr, opaque_key,
 				       desc_idx, *post_ptr);