vfio: Add cdev_device_open_cnt to vfio_group

vfio: Add cdev_device_open_cnt to vfio_group

This is for counting the devices that are opened via the cdev path. This
count is increased and decreased by the cdev path. The group path checks
it to achieve exclusion with the cdev path. With this, only one path
(group path or cdev path) will claim DMA ownership. This avoids scenarios
in which devices within the same group may be opened via different paths.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Terrence Xu <terrence.xu@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Tested-by: Yanting Jiang <yanting.jiang@intel.com>
Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Tested-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20230718135551.6592-9-yi.l.liu@intel.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>