iommufd: Propagate allocation failure in iommufd_veventq_deliver_fetch()

iommufd: Propagate allocation failure in iommufd_veventq_deliver_fetch()

When the kzalloc_obj() fails in iommufd_veventq_deliver_fetch(), it returns
NULL, falsely advertising to userspace that the queue is empty.

Propagate the -ENOMEM properly to the caller.

Fixes: e36ba5ab808e ("iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC")
Link: https://patch.msgid.link/r/25d29feac909e36f78c145fa99ef2d4cb7a415da.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>