scsi: scsi_debug: Fix sdebug_blk_mq_poll() in_use_bm bitmap use

The in_use_bm bitmap of struct sdebug_queue should be accessed under
protection of the qc_lock spinlock. Make sure that this lock is taken
before calling find_first_bit() at the beginning of the function
sdebug_blk_mq_poll().

Link: https://lore.kernel.org/r/20220404045547.579887-1-damien.lemoal@opensource.wdc.com
Fixes: 3fd07aec

 ("scsi: scsi_debug: Fix qc_lock use in sdebug_blk_mq_poll()")
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>