FIX: read_superblock_header() freed buffer before copied

Reported by an Android developer.
Thanks.

I don't see why I did such a mistake..
add flush_supported = true but version >= 3.6 only

dm-cache and dm-thin also turned this flag on.
dm-writeboost also should do so.
modified scripts
Changed TODO
mkdir Documentation/
FIX: don't free buffer before memcpy
make metablock_device size 16B and segment_size_order is limited to 10

Don't straddle two sector.
FIX: endif for a include guard placed at the end
Merge branch 'feature/metablock-16b' into develop
Shape up recovery algorithm

length and lap in semgent_header_device
are removed.
the lap can be calculated from global_id.

and FIX
segment_header_device's global_id should be
changed into CPU's format by le64_to_cpu().
One place missed the code and then be fixed.