vfio-pci: Fix vfio_pci_ioeventfd() to return int

This only returns 0 or -ERRNO, it should return int like all the other
ioctl dispatch functions.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/1-v2-0f9e632d54fb+d6-vfio_ioctl_split_jgg@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>