scsi: libsas: use flush_workqueue to process disco events synchronously

Now we are processing sas event and discover event in different
workqueues.  It's safe to wait the discover event done in the sas event
work. Use flush_workqueue() to insure the disco and revalidate events
processed synchronously so that the whole discover and revalidate
process will not be interrupted by other events.
Signed-off-by: Jason Yan <yanaijie@huawei.com>
CC: John Garry <john.garry@huawei.com>
CC: Johannes Thumshirn <jthumshirn@suse.de>
CC: Ewan Milne <emilne@redhat.com>
CC: Christoph Hellwig <hch@lst.de>
CC: Tomas Henzl <thenzl@redhat.com>
CC: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/libsas/sas_port.c

@@ -192,6 +192,7 @@ static void sas_form_port(struct asd_sas_phy *phy)
 		si->dft->lldd_port_formed(phy);
 
 	sas_discover_event(phy->port, DISCE_DISCOVER_DOMAIN);
+	flush_workqueue(sas_ha->disco_q);
 }
 
 /**
@@ -277,6 +278,9 @@ void sas_porte_broadcast_rcvd(struct work_struct *work)
 
 	SAS_DPRINTK("broadcast received: %d\n", prim);
 	sas_discover_event(phy->port, DISCE_REVALIDATE_DOMAIN);
+
+	if (phy->port)
+		flush_workqueue(phy->port->ha->disco_q);
 }
 
 void sas_porte_link_reset_err(struct work_struct *work)