iommufd: Reject invalid read count in iommufd_fault_fops_read()

iommufd: Reject invalid read count in iommufd_fault_fops_read()

The read count must be large enough to hold one fault or a group's faults.

iommufd_fault_fops_read() does not validate the count, but returns 0 as if
the read had succeeded while leaving the pending fault in the queue.

Return -EINVAL in the undersize cases.

Fixes: 07838f7fd529 ("iommufd: Add iommufd fault object")
Link: https://patch.msgid.link/r/85c118a606fbedc5c132a1f5ec223a5ba23b92d2.1780343944.git.nicolinc@nvidia.com
Cc: stable@vger.kernel.org
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>