Fix decrement_size RPC arguments

`decr_size` and `trunc_data` RPCs were using the same input parameters:

 - `uint64 fuid`
 - `string path`
 - `uint64 length`

`Decr_size` needs `path` but not `fuid` while for `trunc_data` is the
opposite.

Thus we need to use different struct type to store input parameters for
each of those RPCs.