[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/isci/host.c

@@ -650,15 +650,13 @@ static void isci_host_start_complete(struct isci_host *ihost, enum sci_status co
 
 int isci_host_scan_finished(struct Scsi_Host *shost, unsigned long time)
 {
-	struct isci_host *ihost = SHOST_TO_SAS_HA(shost)->lldd_ha;
+	struct sas_ha_struct *ha = SHOST_TO_SAS_HA(shost);
+	struct isci_host *ihost = ha->lldd_ha;
 
 	if (test_bit(IHOST_START_PENDING, &ihost->flags))
 		return 0;
 
-	/* todo: use sas_flush_discovery once it is upstream */
-	scsi_flush_work(shost);
-
-	scsi_flush_work(shost);
+	sas_drain_work(ha);
 
 	dev_dbg(&ihost->pdev->dev,
 		"%s: ihost->status = %d, time = %ld\n",