iommufd: Balance veventq->num_events inc/dec

iommufd_veventq_fops_read() decrements veventq->num_events when a vevent
is read out. However, the report path ony increments veventq->num_events
for normal events. To be balanced, make the read path decrement num_events
only for normal vevents.

Fixes: e36ba5a ("iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC")
Link: https://patch.msgid.link/r/20250324120034.5940-3-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>

Author: yiliu1765

drivers/iommu/iommufd/eventq.c

@@ -385,7 +385,8 @@ static ssize_t iommufd_veventq_fops_read(struct file *filep, char __user *buf,
 			break;
 		}
 		spin_lock(&eventq->lock);
-		veventq->num_events--;
+		if (!vevent_for_lost_events_header(cur))
+			veventq->num_events--;
 		spin_unlock(&eventq->lock);
 		done += cur->data_len;
 		kfree(cur);