x86/iommu: adjust type in arch_iommu_hwdom_init()

The 'i' iterator index stores a PDX, not a PFN, and hence the initial
assignation of start (which stores a PFN) needs a conversion from PFN
to PDX.

This is harmless currently, as the PDX compression skips the bottom
MAX_ORDER bits which cover the low 1MB, but still do the conversion
from PDX to PFN for type correctness.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/drivers/passthrough/x86/iommu.c

@@ -406,7 +406,7 @@ void __hwdom_init arch_iommu_hwdom_init(struct domain *d)
      */
     start = paging_mode_translate(d) ? PFN_DOWN(MB(1)) : 0;
 
-    for ( i = start, count = 0; i < top; )
+    for ( i = pfn_to_pdx(start), count = 0; i < top; )
     {
         unsigned long pfn = pdx_to_pfn(i);
         unsigned int perms = hwdom_iommu_map(d, pfn, max_pfn);