scsi: scsi_debug: Drop sdebug_dev_info.num_in_q

In schedule_resp(), under certain conditions we check whether the
per-device queue is full (num_in_q == queue depth - 1) and we may inject a
"task set full" (TSF) error if it is.

However how we read num_in_q is racy - many threads may see the same "queue
is full" value (and also issue a TSF).

There is per-queue locking in reading per-device num_in_q, but that would
not help.

Replace how we read num_in_q at this location with a call to
scsi_device_busy(). Calling scsi_device_busy() is likewise racy (as reading
num_in_q), so nothing lost or gained. Calling scsi_device_busy() is also
slow as it needs to read all bits in the per-device budget bitmap, but we
can live with that since we're just a simulator and it's only under a
certain configs which we would see this.

Also move the "task set full" print earlier as it would only be called now
under this condition. However, previously it may not have been called -
like returning early - but keep it simple and always call it.

At this point we can drop sdebug_dev_info.num_in_q - it is difficult to
maintain properly and adds extra normal case command processing.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Link: https://lore.kernel.org/r/20230313093114.1498305-10-john.g.garry@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>