[SCSI] Fix scsi_add_device() for async scanning

I had thought that all drivers which didn't call scsi_scan_host()
called scsi_scan_target().  Some, such as sbp2, mptsas and libata-scsi,
call scsi_add_device() or __scsi_add_device().  We just need to wait
for the currently executing async scans to complete first.  This is the
same code that's in scsi_scan_target(), except that we have to return
an error instead of void when we're declining to scan at all.
Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

drivers/scsi/scsi_scan.c

@@ -1453,6 +1453,12 @@ struct scsi_device *__scsi_add_device(struct Scsi_Host *shost, uint channel,
 	struct device *parent = &shost->shost_gendev;
 	struct scsi_target *starget;
 
+	if (strncmp(scsi_scan_type, "none", 4) == 0)
+		return ERR_PTR(-ENODEV);
+
+	if (!shost->async_scan)
+		scsi_complete_async_scans();
+
 	starget = scsi_alloc_target(parent, channel, id);
 	if (!starget)
 		return ERR_PTR(-ENOMEM);