iommu/amd: Fix memory leak in free_pagetable

The IOMMU pagetables can have up to 6 levels, but the code
in free_pagetable() only releases the first 3 levels. Fix
this leak by releasing all levels.

Reported-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>