scsi: iscsi: Flush block work before unblock

We set the max_active iSCSI EH works to 1, so all work is going to execute
in order by default. However, userspace can now override this in sysfs. If
max_active > 1, we can end up with the block_work on CPU1 and
iscsi_unblock_session running the unblock_work on CPU2 and the session and
target/device state will end up out of sync with each other.

This adds a flush of the block_work in iscsi_unblock_session.

Link: https://lore.kernel.org/r/20210525181821.7617-17-michael.christie@oracle.com
Fixes: 1d726aa6 ("scsi: iscsi: Optimize work queue flush use")
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>

drivers/scsi/scsi_transport_iscsi.c

@@ -1969,6 +1969,8 @@ static void __iscsi_unblock_session(struct work_struct *work)
  */
 void iscsi_unblock_session(struct iscsi_cls_session *session)
 {
+	flush_work(&session->block_work);
+
 	queue_work(iscsi_eh_timer_workq, &session->unblock_work);
 	/*
 	 * Blocking the session can be done from any context so we only