scsi: scsi_debug: Refine sdebug_blk_mq_poll()

Refine the sdebug_blk_mq_poll() function so it only takes the spinlock on
the queue when it can see one or more requests with the in_use bitmap flag
set.

Link: https://lore.kernel.org/r/20220109012853.301953-5-dgilbert@interlog.com
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>