fix(1902): correct the skb len

Fix #1902

When we contruct frames for HTTP/2 response and
reuse the same skbs from the backend (HTTP/1), we forget to adjust
the skb size according to the trailer headers (if exist) size, then it
comes with extra partial HTTP/1 response to the client. Then, the client
will send a `GOAWAY` to temepesta due to wrong frame size, and tempesta
sends back the `TCP RESET`.