BUG/MINOR: haterm: fix the response body phase on short writes

BUG/MINOR: haterm: fix the response body phase on short writes

hstream_add_ff_data() computed the offset of the next body byte as
(req_size - len) % modulo, where <len> is the length offered by the mux
for that call. A short write or a partially filled output buffer makes it
smaller than the number of bytes still to send, so the offset no longer
matched what had already been emitted: the repeating pattern jumped and
the peer received a corrupted body, which happened as soon as a proxy read
the response with its own buffer boundaries. Use hs->to_write instead.

Introduced with ecf36f2ca ("MEDIUM: haterm: Add support for 0-copy data
forwading and option to disable it"), present only in the development
branch, so no backport is needed.