scsi: iscsi: Add task completion helper

This adds a helper to detect if a cmd has completed but is not yet freed.

Link: https://lore.kernel.org/r/20210525181821.7617-2-michael.christie@oracle.com


Reviewed-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>