iommufd/fault: Use a separate spinlock to protect fault->deliver list

iommufd/fault: Use a separate spinlock to protect fault->deliver list

The fault->mutex serializes the fault read()/write() fops and the
iommufd_fault_auto_response_faults(), mainly for fault->response. Also, it
was conveniently used to fence the fault->deliver in poll() fop and
iommufd_fault_iopf_handler().

However, copy_from/to_user() may sleep if pagefaults are enabled. Thus,
they could take a long time to wait for user pages to swap in, blocking
iommufd_fault_iopf_handler() and its caller that is typically a shared IRQ
handler of an IOMMU driver, resulting in a potential global DOS.

Instead of reusing the mutex to protect the fault->deliver list, add a
separate spinlock, nested under the mutex, to do the job.
iommufd_fault_iopf_handler() would no longer be blocked by
copy_from/to_user().

Add a free_list in iommufd_auto_response_faults(), so the spinlock can
simply fence a fast list_for_each_entry_safe routine.

Provide two deliver list helpers for iommufd_fault_fops_read() to use:
 - Fetch the first iopf_group out of the fault->deliver list
 - Restore an iopf_group back to the head of the fault->deliver list

Lastly, move the mutex closer to the response in the fault structure,
and update its kdoc accordingly.

Fixes: 07838f7fd529 ("iommufd: Add iommufd fault object")
Link: https://patch.msgid.link/r/20250117192901.79491-1-nicolinc@nvidia.com
Cc: stable@vger.kernel.org
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.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>