scsi: scsi_debug: Change shost list lock to a mutex

The shost list lock, sdebug_host_list_lock, is a spinlock. We would only
lock in non-atomic context in this driver, so use a mutex instead, which is
friendlier if we need to schedule when iterating.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20230327074310.1862889-4-john.g.garry@oracle.com
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>