make checkpointing work on ext4

Test: Test script passes

Change-Id: Iafa337947f2fd456aa692ecb112ccc56638f7947
diff --git a/rootdir/init.rc b/rootdir/init.rc
index f39ea7c..2a70a4a 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -400,6 +400,7 @@
 
     # Make sure we have the device encryption key.
     start vold
+    exec - system system -- /system/bin/vdc checkpoint prepareDriveForCheckpoint /data
     installkey /data
 
     # Start bootcharting as soon as possible after the data partition is