Btrfs: compat code fixes

The btrfs git kernel trees is used to build a standalone tree for
compiling against older kernels.  This commit makes the standalone tree
work with 2.6.27

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index c8dcb47..9816522 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -26,7 +26,8 @@
 #include <linux/buffer_head.h> // for block_sync_page
 #include <linux/workqueue.h>
 #include <linux/kthread.h>
-# include <linux/freezer.h>
+#include <linux/freezer.h>
+#include "compat.h"
 #include "crc32c.h"
 #include "ctree.h"
 #include "disk-io.h"