iommufd: update outdated comment for renamed iommufd_hw_pagetable_alloc()

iommufd: update outdated comment for renamed iommufd_hw_pagetable_alloc()

The function iommufd_hw_pagetable_alloc() was renamed to
iommufd_hwpt_paging_alloc() by commit 89db31635c87
("iommufd: Derive iommufd_hwpt_paging from
iommufd_hw_pagetable").  Update the stale reference in
iommufd_device_auto_get_domain().

Link: https://patch.msgid.link/r/20260321105759.6832-1-kexinsun@smail.nju.edu.cn
Assisted-by: unnamed:deepseek-v3.2 coccinelle
Signed-off-by: Kexin Sun <kexinsun@smail.nju.edu.cn>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>