scsi: scsi_debug: Only allow sdebug_max_queue be modified when no shosts

scsi: scsi_debug: Only allow sdebug_max_queue be modified when no shosts

The shost->can_queue value is initially used to set per-HW queue context
tag depth in the block layer. This ensures that the shost is not sent too
many commands which it can deal with. However lowering sdebug_max_queue
separately means that we can easily overload the shost, as in the following
example:

$ cat /sys/bus/pseudo/drivers/scsi_debug/max_queue
192
$ cat /sys/class/scsi_host/host0/can_queue
192
$ echo 100 > /sys/bus/pseudo/drivers/scsi_debug/max_queue
$ cat /sys/class/scsi_host/host0/can_queue
192
$ fio --filename=/dev/sda --direct=1 --rw=read --bs=4k --iodepth=256
--runtime=1200 --numjobs=10 --time_based --group_reporting
--name=iops-test-job --eta-newline=1 --readonly    --ioengine=io_uring
--hipri --exitall_on_error
iops-test-job: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=io_uring, iodepth=256
...
fio-3.28
Starting 10 processes
[  111.269885] scsi_io_completion_action: 400 callbacks suppressed
[  111.269885] blk_print_req_error: 400 callbacks suppressed
[  111.269889] I/O error, dev sda, sector 440 op 0x0:(READ) flags 0x1200000 phys_seg 1 prio class 2
[  111.269892] sd 0:0:0:0: [sda] tag#132 FAILED Result: hostbyte=DID_ABORT driverbyte=DRIVER_OK cmd_age=0s
[  111.269897] sd 0:0:0:0: [sda] tag#132 CDB: Read(10) 28 00 00 00 01 68 00 00 08 00
[  111.277058] I/O error, dev sda, sector 360 op 0x0:(READ) flags 0x1200000 phys_seg 1 prio class 2

[...]

Ensure that this cannot happen by allowing sdebug_max_queue be modified
only when we have no shosts. As such, any shost->can_queue value will match
sdebug_max_queue, and sdebug_max_queue cannot be modified separately.

Since retired_max_queue is no longer set, remove support.

Continue to apply the restriction that sdebug_host_max_queue cannot be
modified when sdebug_host_max_queue is set. Adding support for that would
mean extra code, and no one has complained about this restriction
previously.

A command like the following may be used to remove a shost:
echo -1 > /sys/bus/pseudo/drivers/scsi_debug/add_host

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20230327074310.1862889-11-john.g.garry@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>