fsp_srv: Respect write length in Write()

Previously we were just copying the data whole-sale, even if the length
was less than the total data size. This effectively makes the
actual_data vector useless, which is likely not intended.

Instead, amend this to only copy the given length amount of data.

At the same time, we can avoid zeroing out the data before using it by
passing iterators to the constructor instead of a size.