msm8660: Add length offset for data partition

 * When using "footer" style encryption metadata, we need to specify an
   offset on the length of the partition as well in order to make room
   for said footer. This was previously handled in the old
   recovery.fstab format.
 * Should fix the issue where the system explodes after a wipe.

Change-Id: Ie0686e047d2fb8b72eb91ab00319354eb5cfe6bf