igb: cleanup tx dma so map & unmap use matching calls

The igb driver was using map_single to map the skbs and then unmap_page to
unmap them.  This update changes that so instead uses skb_dma_map and
skb_dma_unmap.

In addition the next_to_watch member of the buffer_info struct was being
set uneccesarily.  I removed the spots where it was set without being needed.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>