iommufd: Assert devices_lock for iommufd_hw_pagetable_has_group()

The hwpt->devices list is locked by this, make it clearer.

Link: https://lore.kernel.org/r/1-v3-ae9c2975a131+2e1e8-iommufd_hwpt_jgg@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>