iommufd: Correct IOMMU_HWPT_ALLOC_NEST_PARENT description

The IOMMU_HWPT_ALLOC_NEST_PARENT flag is used to allocate a HWPT. Though
a HWPT holds a domain in the core structure, it is still quite confusing
to describe it using "domain" in the uAPI kdoc. Correct it to "HWPT".

Fixes: 4ff54216 ("iommufd: Support allocating nested parent domain")
Link: https://lore.kernel.org/r/20231017181552.12667-1-nicolinc@nvidia.com


Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>