scsi: iscsi: Register sysfs for iscsi workqueue

This patch enables setting cpu affinity through "cpumask" for iscsi
workqueues (iscsi_q_xx and iscsi_eh), so as to get performance isolation.

The max number of active worker was changed form 1 to 2, because "cpumask"
of ordered workqueue isn't allowed to change.

Link: https://lore.kernel.org/r/20200505011908.15538-1-bob.liu@oracle.com
Reviewed-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>