forcedeth: fix dma api mismatches

forcedeth doesnt use properly dma api in its tx completion path
and in nv_loopback_test()

pci_map_single() should be paired with pci_unmap_single()
pci_map_page() should be paired with pci_unmap_page()

forcedeth xmit path uses pci_map_single() & pci_map_page(),
but tx completion path only uses pci_unmap_single()

nv_loopback_test() uses pci_map_single() & pci_unmap_page()

Add a dma_single field in struct nv_skb_map, and
define a helper function nv_unmap_txskb

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>