scsi: core: Cap scsi_host cmd_per_lun at can_queue

The sysfs handling function sdev_store_queue_depth() enforces that the sdev
queue depth cannot exceed shost can_queue. The initial sdev queue depth
comes from shost cmd_per_lun. However, the LLDD may manually set
cmd_per_lun to be larger than can_queue, which leads to an initial sdev
queue depth greater than can_queue.

Such an issue was reported in [0], which caused a hang. That has since been
fixed in commit fc09acb7

 ("scsi: scsi_debug: Fix cmd_per_lun, set to
max_queue").

Stop this possibly happening for other drivers by capping shost cmd_per_lun
at shost can_queue.

[0] https://lore.kernel.org/linux-scsi/YHaez6iN2HHYxYOh@T590/

Link: https://lore.kernel.org/r/1621434662-173079-1-git-send-email-john.garry@huawei.com
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>