tcp: write option length correctly
tcp: write option length correctly

As pointed out by Tomasz and again by Asias, we should write len, not kind,
in len's slot.

avikivity/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,cloudius-systems/seastar,kjniemi/seastar,avikivity/seastar,dreamsxin/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,kjniemi/seastar,kjniemi/seastar,dreamsxin/seastar