vfio/mdev: add mdev available instance checking to the core

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: Jason Gunthorpe <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Kevin Tian <[email protected]>
Reviewed-by: Kirti Wankhede <[email protected]>
Reviewed-by: Eric Farman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alex Williamson <[email protected]>

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