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fops->open and fops->release are always paired.  Use simple atomic bit
ops ot indicate if the device is opened instead of a count that can
only be 0 and 1 and a useless cmpxchg loop in ublk_ch_release.

Also don't bother clearing file->private_data is the file is about to
be freed anyway.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220721130916.1869719-5-hch@lst.de

Signed-off-by: Jens Axboe <axboe@kernel.dk>