iommu: Fix two issues in iommu_copy_struct_from_user()

In the Linux kernel, the following vulnerability has been resolved: iommu: Fix two issues in iommu_copy_struct_from_user() In the review for iommu_copy_struct_to_user() helper, Matt pointed out that a NULL pointer should be rejected prior to dereferencing it: https://lore.kernel.org/all/86881827-8E2D-461C-BDA3-FA8FD14C343C@nvidia.com And Alok pointed out a typo at the same time: https://lore.kernel.org/all/480536af-6830-43ce-a327-adbd13dc3f1d@oracle.com Since both issues were copied from iommu_copy_struct_from_user(), fix them first in the current header.