[SCSI] libsas: introduce sas_drain_work()

When an lldd invokes ->notify_port_event() it can trigger a chain of libsas
events to:

  1/ form the port and find the direct attached device

  2/ if the attached device is an expander perform domain discovery

A call to flush_workqueue() will only flush the initial port formation work.
Currently libsas users need to call scsi_flush_work() up to the max depth of
chain (which will grow from 2 to 3 when ata discovery is moved to its own
discovery event).  Instead of open coding multiple calls switch to use
drain_workqueue() to flush sas work.

drain_workqueue() does not handle new work submitted during the drain so
libsas needs a bit of infrastructure to hold off unchained work submissions
while a drain is in flight.  A lldd ->notify() event is considered 'unchained'
while a sas_discover_event() is 'chained'.  As Tejun notes:

  "For now, I think it would be best to add private wrapper in libsas to
   support deferring unchained work items while draining."

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/pm8001/pm8001_sas.c

@@ -256,12 +256,14 @@ void pm8001_scan_start(struct Scsi_Host *shost)
 
 int pm8001_scan_finished(struct Scsi_Host *shost, unsigned long time)
 {
+	struct sas_ha_struct *ha = SHOST_TO_SAS_HA(shost);
+
 	/* give the phy enabling interrupt event time to come in (1s
 	* is empirically about all it takes) */
 	if (time < HZ)
 		return 0;
 	/* Wait for discovery to finish */
-	scsi_flush_work(shost);
+	sas_drain_work(ha);
 	return 1;
 }