Staging: rtl8192e: fix IOMMU memory leak

Unmap the rx buffer before mapping the new one in rtl8192_rx.

Failing to do so quickly exhausts the IOMMU memory during downloads:

[...] DMA: Out of SW-IOMMU space for 9100 bytes at device ...

Using "iommu=off mem=4g" also fixes the problem because
then pci_map_single does not allocate memory.

Tested on my personal laptop with a RTL8192E device. Without this
patch the kernel quickly runs out of IOMMU memory (downloading 5 MB
of data is sufficient to trigger it), with this patch applied
I haven't experienced any issues so far.

Signed-off-by: Daniel Lichtenberger <daniel.lichtenberger@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/rtl8192e/r8192E_core.c

@@ -5829,6 +5829,9 @@ static void rtl8192_rx(struct net_device *dev)
                     }
                 }
 
+		pci_unmap_single(priv->pdev, *((dma_addr_t *) skb->cb),
+			priv->rxbuffersize, PCI_DMA_FROMDEVICE);
+
                 skb = new_skb;
                 priv->rx_buf[priv->rx_idx] = skb;
                 *((dma_addr_t *) skb->cb) = pci_map_single(priv->pdev, skb_tail_pointer(skb), priv->rxbuffersize, PCI_DMA_FROMDEVICE);