sfc: Fix efx_rx_buf_offset() for recycled pages

This bug fix is only for stable branches older than 3.10.  The bug was
fixed upstream by commit 2768935a ('sfc: reuse pages to avoid DMA
mapping/unmapping costs'), but that change is totally unsuitable for
stable.

Commit b590ace0 ('sfc: Fix efx_rx_buf_offset() in the presence of
swiotlb') added an explicit page_offset member to struct
efx_rx_buffer, which must be set consistently with the u.page and
dma_addr fields.  However, it failed to add the necessary assignment
in efx_resurrect_rx_buffer().  It also did not correct the calculation
of efx_rx_buffer::dma_addr in efx_resurrect_rx_buffer(), which assumes
that DMA-mapping a page will result in a page-aligned DMA address
(exactly what swiotlb violates).

Add the assignment of efx_rx_buffer::page_offset and change the
calculation of dma_addr to make use of it.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: David Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/ethernet/sfc/rx.c

@@ -311,8 +311,9 @@ static void efx_resurrect_rx_buffer(struct efx_rx_queue *rx_queue,
 
 	index = rx_queue->added_count & rx_queue->ptr_mask;
 	new_buf = efx_rx_buffer(rx_queue, index);
-	new_buf->dma_addr = rx_buf->dma_addr ^ (PAGE_SIZE >> 1);
 	new_buf->u.page = rx_buf->u.page;
+	new_buf->page_offset = rx_buf->page_offset ^ (PAGE_SIZE >> 1);
+	new_buf->dma_addr = state->dma_addr + new_buf->page_offset;
 	new_buf->len = rx_buf->len;
 	new_buf->flags = EFX_RX_BUF_PAGE;
 	++rx_queue->added_count;