scsi: scsi_debug: Stop using READ/WRITE_ONCE() when accessing sdebug_defer.defer_t

scsi: scsi_debug: Stop using READ/WRITE_ONCE() when accessing sdebug_defer.defer_t

Using READ/WRITE_ONCE() means that the read or write is not torn by the
compiler.

READ/WRITE_ONCE() is always used when accessing sdebug_defer.defer_t.

However, we also guard the access in a spinlock when accessing that
member, and spinlock already guarantees no tearing, so stop using
READ/WRITE_ONCE().

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://patch.msgid.link/20251113133645.2898748-3-john.g.garry@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>