Add prefetches of the skb and the next rx descriptor to speed up rx path.

Use prefetchw() for the skb [suggested by Eric Dumazet].

The rx descriptor is in skb->data which is mapped for streaming mode DMA.
Eric Dumazet pointed out that we should not prefetch the data before
dma_sync.  So we prefetch only if dma_sync is no_op on the system.
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>