The data blocks are identical for files using traditional direct/indirect
block allocation scheme and extent trees, thus this code part can be
common. Only the code to deallocate the indirect blocks to record the
used blocks has to be seperate, respectively the code to release extent
tree index blocks.

Actually the code to release the extent tree index blocks is still missing,
but at least add a FIXME at the appropriate place.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>