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.