ANDROID: Incremental fs: Protect get_fill_block, and add a field

Since INCFS_IOC_GET_FILLED_BLOCKS potentially leaks information about usage
patterns, and is only useful to someone filling the file, best protect it in
the same way as INCFS_IOC_FILL_BLOCKS.

Add useful field data_block_out as well

Test: incfs_test passes
Bug: 152983639
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I126a8cf711e56592479093e9aadbfd0e7f700752