iommufd: Generalize iopt_pages address

iommufd: Generalize iopt_pages address

The starting address in iopt_pages is currently a __user *uptr.
Generalize to allow other types of addresses.  Refactor iopt_alloc_pages()
and iopt_map_user_pages() into address-type specific and common functions.

Link: https://patch.msgid.link/r/1729861919-234514-4-git-send-email-steven.sistare@oracle.com
Suggested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>