virtio_net: use correct accessors for scatterlists

Without this fix, virtio_net makes incorrect usage of scatterlists. It sets
the end of the scatterlist chain after the first element, despite the fact
that more entries come after it.

If you try to run dma_map_sg() on one of the scatterlists given to you by
add_buf(), you will get a null pointer oops.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>