defxx: Discard DMA maps on buffer deallocation

Prearranged receive DMA bounce buffer mappings are not released in the
card reboot/shutdown path.  That does not affect frame reception, but
probably explains the random segmentation fault I observed the other day
on interface shutdown.  Card is rebooted as required by the spec in the
process of ring fault recovery when a PC Trace signal has been received.

This change fixes the problem in an obvious manner.
Reported-by: Robert Coerver <Robert.Coerver@ll.mit.edu>
Tested-by: Robert Coerver <Robert.Coerver@ll.mit.edu>
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/fddi/defxx.c

@@ -3447,8 +3447,13 @@ static void dfx_rcv_flush( DFX_board_t *bp )
 		{
 			struct sk_buff *skb;
 			skb = (struct sk_buff *)bp->p_rcv_buff_va[i+j];
-			if (skb)
+			if (skb) {
+				dma_unmap_single(bp->bus_dev,
+						 bp->descr_block_virt->rcv_data[i+j].long_1,
+						 PI_RCV_DATA_K_SIZE_MAX,
+						 DMA_FROM_DEVICE);
 				dev_kfree_skb(skb);
+			}
 			bp->p_rcv_buff_va[i+j] = NULL;
 		}