scsi: scsi_debug: Use scsi_block_requests() to block queues

The feature to block queues is quite dubious, since it races with in-flight
IO. Indeed, it seems unnecessary for block queues for any times we do so.

Anyway, to keep the same behaviour, use standard SCSI API to stop IO being
sent - scsi_{un}block_requests().

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20230327074310.1862889-6-john.g.garry@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>