scsi: scsi_debug: Fix in_use bitmap corruption

scsi: scsi_debug: Fix in_use bitmap corruption

Heavy testing indicates the irqsave() spinlock around the __set_bit() is
insufficient to stop following clear_bit() calls being rarely applied
out-of-order. Also the nearby failed kzalloc() path leading to
SCSI_MLQUEUE_HOST_BUSY does not properly undo the in_use bitmap and
num_in_q, fix.

Link: https://lore.kernel.org/r/20200702145355.522283-1-dgilbert@interlog.com
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>