intel-iommu: Fix leaks in pagetable freeing

At best the current code only seems to free the leaf pagetables and
the root.  If you're unlucky enough to have a large gap (like any
QEMU guest with more than 3G of memory), only the first chunk of leaf
pagetables are freed (plus the root).  This is a massive memory leak.
This patch re-writes the pagetable freeing function to use a
recursive algorithm and manages to not only free all the pagetables,
but does it without any apparent performance loss versus the current
broken version.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: stable@vger.kernel.org
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>