iommufd: Reject invalid read count in iommufd_veventq_fops_read()

iommufd: Reject invalid read count in iommufd_veventq_fops_read()

The read count must be large enough to hold a vEVENT header. For a normal
vEVENT, it must also hold the trailing data following the header.

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

Return -EINVAL in both undersize cases.

Fixes: e36ba5ab808e ("iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC")
Link: https://patch.msgid.link/r/e1111adcc8a8882fbfd84accd6674dc846dc5689.1780343944.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>