iommufd: Abstract an iommufd_eventq from iommufd_fault

iommufd: Abstract an iommufd_eventq from iommufd_fault

The fault object was designed exclusively for hwpt's IO page faults (PRI).
But its queue implementation can be reused for other purposes too, such as
hardware IRQ and event injections to user space.

Meanwhile, a fault object holds a list of faults. So it's more accurate to
call it a "fault queue". Combining the reusing idea above, abstract a new
iommufd_eventq as a common structure embedded into struct iommufd_fault,
similar to hwpt_paging holding a common hwpt.

Add a common iommufd_eventq_ops and iommufd_eventq_init to prepare for an
IOMMUFD_OBJ_VEVENTQ (vIOMMU Event Queue).

Link: https://patch.msgid.link/r/e7336a857954209aabb466e0694aab323da95d90.1741719725.git.nicolinc@nvidia.com
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>