iommu/vt-d: only unmap mapped entries

Current unmap implementation unmaps the entire area covered by the IOVA
range, which is a power-of-2 aligned region. The corresponding map,
however, only maps those pages originally mapped by the user. This
discrepancy can lead to unmapping of already unmapped entries, which is
unneeded work.

With this patch, only mapped pages are unmapped. This is also a baseline
for a map/unmap implementation based on IOVAs and not iova structures,
which will allow caching.

Signed-off-by: Omer Peleg <omer@cs.technion.ac.il>
[mad@cs.technion.ac.il: rebased and reworded the commit message]
Signed-off-by: Adam Morrison <mad@cs.technion.ac.il>
Reviewed-by: Shaohua Li <shli@fb.com>
Reviewed-by: Ben Serebrin <serebrin@google.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>