iommu: Replace iommu_group_device_count() with list_count_nodes()

iommu: Replace iommu_group_device_count() with list_count_nodes()

No reason to wrapper a standard function, just call the library directly.

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/1-v5-1b99ae392328+44574-iommu_err_unwind_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>