scsi: scsi_debug: Strengthen defer_t accesses

Use READ_ONCE() and WRITE_ONCE() macros when accessing the
sdebug_defer::defer_t value.

Link: https://lore.kernel.org/r/20220109012853.301953-3-dgilbert@interlog.com


Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>