Use right LBA to get the backed up WorkSpace in spare block.

Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14398 6f19259b-4bc3-4df7-8a09-765794883524