Fix non-ab fuse ota fail bug

Fix the 'Calculate the offset to last partial block' logic in
ReadBlockAligndData. If file_size % fuse_block_size_ < source_block_size_,
the origin map logic will get a wrong block address of last block.
This bug leads to the non-ab fuse ota fail.

Test: compare the md5 of origin pkg and pkg reading from fuse.
Change-Id: Ic5132a7e9919cc2e91390231be342b97421ea23e