scsi_dh: kill struct scsi_dh_data

Add a ->handler and a ->handler_data field to struct scsi_device and kill
this indirection.  Also move struct scsi_device_handler to scsi_dh.h so that
changes to it don't require rebuilding every SCSI LLDD.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 882864f..cbfc599 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -31,6 +31,7 @@
 #include <scsi/scsi_driver.h>
 #include <scsi/scsi_eh.h>
 #include <scsi/scsi_host.h>
+#include <scsi/scsi_dh.h>
 
 #include <trace/events/scsi.h>
 
@@ -1248,9 +1249,8 @@
 {
 	struct scsi_cmnd *cmd = req->special;
 
-	if (unlikely(sdev->scsi_dh_data && sdev->scsi_dh_data->scsi_dh
-			 && sdev->scsi_dh_data->scsi_dh->prep_fn)) {
-		int ret = sdev->scsi_dh_data->scsi_dh->prep_fn(sdev, req);
+	if (unlikely(sdev->handler && sdev->handler->prep_fn)) {
+		int ret = sdev->handler->prep_fn(sdev, req);
 		if (ret != BLKPREP_OK)
 			return ret;
 	}