vfio/spapr_tce: Check kzalloc() return when preregistering memory

This adds missing checking for kzalloc() return value.

Fixes: 4b6fad70 ("powerpc/mm/iommu, vfio/spapr: Put pages on VFIO container shutdown")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>