iommufd: Allow pt_id to carry viommu_id for IOMMU_HWPT_ALLOC

iommufd: Allow pt_id to carry viommu_id for IOMMU_HWPT_ALLOC

Now a vIOMMU holds a shareable nesting parent HWPT. So, it can act like
that nesting parent HWPT to allocate a nested HWPT.

Support that in the IOMMU_HWPT_ALLOC ioctl handler, and update its kdoc.

Also, add an iommufd_viommu_alloc_hwpt_nested helper to allocate a nested
HWPT for a vIOMMU object. Since a vIOMMU object holds the parent hwpt's
refcount already, increase the refcount of the vIOMMU only.

Link: https://patch.msgid.link/r/a0f24f32bfada8b448d17587adcaedeeb50a67ed.1730836219.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>