mq-deadline: Make sure to always unlock zones

In case of a failed write request (all retries failed) and when using
libata, the SCSI error handler calls scsi_finish_command(). In the
case of blk-mq this means that scsi_mq_done() does not get called,
that blk_mq_complete_request() does not get called and also that the
mq-deadline .completed_request() method is not called. This results in
the target zone of the failed write request being left in a locked
state, preventing that any new write requests are issued to the same
zone.

Fix this by replacing the .completed_request() method with the
.finish_request() method as this method is always called whether or
not a request completes successfully. Since the .finish_request()
method is only called by the blk-mq core if a .prepare_request()
method exists, add a dummy .prepare_request() method.

Fixes: 5700f69178e9 ("mq-deadline: Introduce zone locking support")
Cc: Hannes Reinecke <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Signed-off-by: Damien Le Moal <[email protected]>
[ bvanassche: edited patch description ]
Signed-off-by: Bart Van Assche <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

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