iommu/vt-d: fix memory leakage caused by commit ea8ea460

Commit ea8ea460 "iommu/vt-d: Clean up and fix page table clear/free
behaviour" introduces possible leakage of DMA page tables due to:
        for (pte = page_address(pg); !first_pte_in_page(pte); pte++) {
                if (dma_pte_present(pte) && !dma_pte_superpage(pte))
                        freelist = dma_pte_list_pagetables(domain, level - 1,
                                                           pte, freelist);
        }

For the first pte in a page, first_pte_in_page(pte) will always be true,
thus dma_pte_list_pagetables() will never be called and leak DMA page
tables if level is bigger than 1.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>