Extract the original `FSNode` passed inside the `ipld.Node` argument and modify
its `Blocksizes` (removing all of them and re-adding the ones that were not
truncated). In contrast, the replaced code was creating a new `FSNode` that was
not preserving some of the features of the original one.

Change `TRUNC_HASH` values in `sharness` that were created with the bug to the
correct values.

License: MIT
Signed-off-by: Lucas Molas <schomatis@gmail.com>