mvpp2: prefetch frame header

When receiving traffic, eth_type_trans() is high up on the perf top list,
because it's the first function which access the packet data.

Move the DMA unmap a bit higher, and put a prefetch just after it, so we
have more time to load the data into the cache.

The packet rate increase is about 14% with a tc drop test: 1620 => 1853 kpps
Signed-off-by: NMatteo Croce <mcroce@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c

@@ -2962,6 +2962,7 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 		dma_sync_single_for_cpu(dev->dev.parent, dma_addr,
 					rx_bytes + MVPP2_MH_SIZE,
 					DMA_FROM_DEVICE);
+		prefetch(data);
 
 		if (bm_pool->frag_size > PAGE_SIZE)
 			frag_size = 0;