Many of the mdev drivers use a simple counter for keeping track of the
available instances. Move this code to the core code and store the counter
in the mdev_parent. Implement it using correct locking, fixing mdpy.

Drivers just provide the value in the mdev_driver at registration time
and the core code takes care of maintaining it and exposing the value in
sysfs.

[hch: count instances per-parent instead of per-type, use an atomic_t
 to avoid taking mdev_list_lock in the show method]
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NKevin Tian <kevin.tian@intel.com>
Reviewed-by: NKirti Wankhede <kwankhede@nvidia.com>
Reviewed-by: NEric Farman <farman@linux.ibm.com>
Link: https://lore.kernel.org/r/20220923092652.100656-15-hch@lst.deSigned-off-by: NAlex Williamson <alex.williamson@redhat.com>