net: tcp: Move the CLOSED state to the front

In enum tcp_state {}, the CLOSED state was put at the last one.
When we do Sequence & Ack validation, we will need to skip the
CLOSED, LISTEN, SYNSENT states. It is easier for coding if we
put the CLOSED to the front, e.g. if state > SYNSENT. And, in
other OSes, the state sequence is normally defined like this.

Signed-off-by: Shrek Wang <inet_eman@outlook.com>