vfio/mtty: Delete mdev_devices_list

vfio/mtty: Delete mdev_devices_list

Dan points out that an error case left things on this list. It is also
missing locking in available_instances_show().

Further study shows the list isn't needed at all, just store the total
ports in use in an atomic and delete the whole thing.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 09177ac91921 ("vfio/mtty: Convert to use vfio_register_group_dev()")
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/0-v1-0bc56b362ca7+62-mtty_used_ports_jgg@nvidia.com
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>