ixgbe: fix unmap length bug

This patch addresses three WARN_ON statements from DMA-API debug code

ixgbe is mapping more than it unmaps, reduce the length of the map call and
remove the "used once" local variable.

found by Joerg Roedel <joerg.roedel@amd.com> in 2.6.30, so is a candidate
for -stable.

in addition, fix missing ->dma = 0 after unmap to prevent double free with
pci_unmap_single

and lastly, don't unmap (half) pages that aren't mapped.

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