iommufd: Initialize the flags of vevent in iommufd_viommu_report_event()

iommufd: Initialize the flags of vevent in iommufd_viommu_report_event()

The vevent->header.flags is not initialized per allocation, hence the
vevent read path may treat the vevent as lost_events_header wrongly.
Use kzalloc() to alloc memory for new vevent.

Fixes: e8e1ef9b77a7 ("iommufd/viommu: Add iommufd_viommu_report_event helper")
Link: https://patch.msgid.link/r/20250324120034.5940-2-yi.l.liu@intel.com
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>