gianfar: fix BUG under load after introduction of skb recycling

Since commit 0fd56bb5be6455d0d42241e65aed057244665e5e ("gianfar:
Add support for skb recycling"), gianfar puts skbuffs that are in
the rx ring back onto the recycle list as-is in case there was a
receive error, but this breaks the following invariant: that all
skbuffs on the recycle list have skb->data = skb->head + NET_SKB_PAD.

The RXBUF_ALIGNMENT realignment done in gfar_new_skb() will be done
twice on skbuffs recycled in this way, causing there not to be enough
room in the skb anymore to receive a full packet, eventually leading
to an skb_over_panic from gfar_clean_rx_ring() -> skb_put().

Resetting the skb->data pointer to skb->head + NET_SKB_PAD before
putting the skb back onto the recycle list restores the mentioned
invariant, and should fix this issue.

Reported-by: Michael Guntsche <mike@it-loops.com>
Tested-by: Michael Guntsche <mike@it-loops.com>
Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/gianfar.c

@@ -1885,8 +1885,17 @@ int gfar_clean_rx_ring(struct net_device *dev, int rx_work_limit)
 
 			if (unlikely(!newskb))
 				newskb = skb;
-			else if (skb)
+			else if (skb) {
+				/*
+				 * We need to reset ->data to what it
+				 * was before gfar_new_skb() re-aligned
+				 * it to an RXBUF_ALIGNMENT boundary
+				 * before we put the skb back on the
+				 * recycle list.
+				 */
+				skb->data = skb->head + NET_SKB_PAD;
 				__skb_queue_head(&priv->rx_recycle, skb);
+			}
 		} else {
 			/* Increment the number of packets */
 			dev->stats.rx_packets++;