gateway: fix wait_forward_request()

We should directly err-return because wi->nr_sent isn't 0 or we'll wrongly goto
again at finish_write.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheepdog 0.5.6

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>