iommu: Fix two issues in iommu_copy_struct_from_user()

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.

Fixes: e9d36c07bb78 ("iommu: Add iommu_copy_struct_from_user helper")
Cc: stable@vger.kernel.org
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Matthew R. Ochs <mochs@nvidia.com>
Link: https://lore.kernel.org/r/20250414191635.450472-1-nicolinc@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>