iommufd/driver: Fix counter initialization for counted_by annotation

iommufd/driver: Fix counter initialization for counted_by annotation

One of the requirements for counted_by annotations is that the counter
member must be initialized before the first reference to the
flexible-array member.

Move the vevent->data_len = data_len; initialization to before the
first access to flexible array vevent->event_data.

Link: https://patch.msgid.link/r/aRL7ZFFqM5bRTd2D@kspp
Cc: stable@vger.kernel.org
Fixes: e8e1ef9b77a7 ("iommufd/viommu: Add iommufd_viommu_report_event helper")
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>