blkio: Introduce blkio controller cgroup interface

o This is basic implementation of blkio controller cgroup interface. This is
  the common interface visible to user space and should be used by different
  IO control policies as we implement those.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/include/linux/iocontext.h b/include/linux/iocontext.h
index eb73632..d61b0b8 100644
--- a/include/linux/iocontext.h
+++ b/include/linux/iocontext.h
@@ -68,6 +68,10 @@
 	unsigned short ioprio;
 	unsigned short ioprio_changed;
 
+#ifdef CONFIG_BLK_CGROUP
+	unsigned short cgroup_changed;
+#endif
+
 	/*
 	 * For request batching
 	 */