[SCSI] fix async probe regression

Commit a7a20d10 "[SCSI] sd: limit the scope of the async probe domain"
moved sd probe work out of reach of wait_for_device_probe().  Allow it
to be synced via scsi_complete_async_scans().
Reported-by: NMeelis Roos <mroos@linux.ee>
Tested-by: NMeelis Roos <mroos@linux.ee>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>

drivers/scsi/scsi_scan.c

@@ -147,7 +147,7 @@ int scsi_complete_async_scans(void)
 
 	do {
 		if (list_empty(&scanning_hosts))
-			return 0;
+			goto out;
 		/* If we can't get memory immediately, that's OK.  Just
 		 * sleep a little.  Even if we never get memory, the async
 		 * scans will finish eventually.
@@ -179,8 +179,11 @@ int scsi_complete_async_scans(void)
 	}
  done:
 	spin_unlock(&async_scan_lock);
-
 	kfree(data);
+
+ out:
+	async_synchronize_full_domain(&scsi_sd_probe_domain);
+
 	return 0;
 }