xen/iommu: x86: Don't try to free page tables is the IOMMU is not enabled

xen/iommu: x86: Don't try to free page tables is the IOMMU is not enabled

When using CONFIG_BIGMEM=y, the page_list cannot be accessed whilst it
is is unitialized. However, iommu_free_pgtables() will be called even if
the domain is not using an IOMMU.

Consequently, Xen will try to go through the page list and deference a
NULL pointer.

Bail out early if the domain is not using an IOMMU.

Fixes: 15bc9a1ef51c ("x86/iommu: add common page-table allocator")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>