igb: fix unmap length bug

driver was mixing NET_IP_ALIGN count bytes in map/unmap calls
unevenly. Only map the bytes that the hardware might dma into

also fix unmap related bug where ->dma was not being cleared
after unmap

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>