sh_eth: Fix padding of short frames on TX

If an skb to be transmitted is shorter than the minimum Ethernet frame
length, we currently set the DMA descriptor length to the minimum but
do not add zero-padding.  This could result in leaking sensitive
data.  We also pass different lengths to dma_map_single() and
dma_unmap_single().

Use skb_padto() to pad properly, before calling dma_map_single().

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>