vfio: Change struct vfio_group::opened from an atomic to bool

This is not a performance path, just use the group_rwsem to protect the
value.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/r/2-v2-d035a1842d81+1bf-vfio_group_locking_jgg@nvidia.com


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