iommufd: Move putting a hwpt to a helper function

Next patch will need to call this from two places.

Link: https://lore.kernel.org/r/8-v8-6659224517ea+532-iommufd_alloc_jgg@nvidia.com


Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>