scsi: iscsi: Register sysfs for workqueue iscsi_destroy

Register sysfs for workqueue iscsi_destroy so that users can set CPU
affinity through "cpumask" for this workqueue to get better isolation in
cloud multi-tenant scenario.

This patch unfolded create_singlethread_workqueue(), added WQ_SYSFS and
drop __WQ_ORDERED_EXPLICIT since __WQ_ORDERED_EXPLICIT workqueue isn't
allowed to change "cpumask".

Link: https://lore.kernel.org/r/20200703051603.1473-1-bob.liu@oracle.com


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