Use the SCSI midlayer helper to traverse the number of outstanding
commands. This also eliminates the last usage for the cmd_list
functionality so we can drop it.

Link: https://lore.kernel.org/r/20200228075318.91255-13-hare@suse.deReviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Balsundar P <balsundar.p@microchip.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>