vfio: Follow the naming pattern for vfio_group_ioctl_unset_container()

Make it clear that this is the body of the ioctl. Fold the locking into
the function so it is self contained like the other ioctls.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/7-v2-0f9e632d54fb+d6-vfio_ioctl_split_jgg@nvidia.com


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