iommu: make map and unmap take a page count, similar to flush

iommu: make map and unmap take a page count, similar to flush

At the moment iommu_map() and iommu_unmap() take a page order rather than a
count, whereas iommu_iotlb_flush() takes a page count rather than an order.
This patch makes them consistent with each other, opting for a page count since
CPU page orders are not necessarily the same as those of an IOMMU.

NOTE: The 'page_count' parameter is also made an unsigned long in all the
      aforementioned functions.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <julien@xen.org>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Wei Liu <wl@xen.org>