IOW, the distance of cutoff from the begining of the branch
(in blocks).

That (and the fact that block just prior to cutoff is guaranteed to
be present) allows to tell whether to free triple indirect block
just by looking at the offset.

While we are at it, using u64 for index in the block is wrong -
those should be unsigned int.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>