iommufd/device: Replace idev->igroup with local variable

iommufd/device: Replace idev->igroup with local variable

With more use of the fields of igroup, use a local vairable instead of
using the idev->igroup heavily.

No functional change expected.

Link: https://patch.msgid.link/r/20250321171940.7213-6-yi.l.liu@intel.com
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>