lightnvm: deprecated OCSSD support and schedule it for removal in Linux 5.15

lightnvm: deprecated OCSSD support and schedule it for removal in Linux 5.15

Lightnvm was an innovative idea to expose more low-level control over SSDs.
But it failed to get properly standardized and remains a non-standarized
extension to NVMe that requires vendor specific quirks for a few now mostly
obsolete SSD devices.  The standardized ZNS command set for NVMe has take
over a lot of the approaches and allows for fully standardized operation.

Remove the Linux code to support open channel SSDs as the few production
deployments of the above mentioned SSDs are using userspace driver stacks
instead of the fairly limited Linux support.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Link: https://lore.kernel.org/r/20210413105257.159260-5-matias.bjorling@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>