block: implement an unprep function corresponding directly to prep

Reviewed-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 5f11608..ee83619 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -85,7 +85,7 @@
 {
 	struct scsi_cmnd *cmd = req->special;
 
-	req->cmd_flags &= ~REQ_DONTPREP;
+	blk_unprep_request(req);
 	req->special = NULL;
 
 	scsi_put_command(cmd);