iommu/vt-d: Use per-cpu IOVA caching

Commit 9257b4a2 ('iommu/iova: introduce per-cpu caching to iova allocation')
introduced per-CPU IOVA caches to massively improve scalability. Use them.

Signed-off-by: Omer Peleg <omer@cs.technion.ac.il>
[mad@cs.technion.ac.il: rebased, cleaned up 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>
[dwmw2: split out VT-d part into a separate patch]
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>