vfio/platform: Use open_device() instead of open coding a refcnt scheme

vfio/platform: Use open_device() instead of open coding a refcnt scheme

Platform simply wants to run some code when the device is first
opened/last closed. Use the core framework and locking for this.  Aside
from removing a bit of code this narrows the locking scope from a global
lock.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/7-v4-9ea22c5e6afb+1adf-vfio_reflck_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>