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/[email protected] And Alok pointed out a typo at the same time: https://lore.kernel.org/all/[email protected] Since both issues were copied from iommu_copy_struct_from_user(), fix them first in the current header.