iommu/amd: Handle large pages correctly in free_pagetable

Make sure that we are skipping over large PTEs while walking
the page-table tree.

Cc: stable@kernel.org
Fixes: 5c34c403 ("iommu/amd: Fix memory leak in free_pagetable")
Signed-off-by: Joerg Roedel <jroedel@suse.de>